On Wednesday, Twitter permanently banned the conservative writer Milo Yiannopoulos as it cracked down on a wave of racist abuse targeting the Ghostbusters actor Leslie Jones.
Twitter also told the Breitbart Tech writer, who is one of the best-known figures among the internet's "alt-right", that his account would not be restored in the future.
The permanent suspension is the social network's strongest response against those who break its abuse and harassment policies, but we know very little about how Twitter decides when to use it.
Twitter doesn't generally comment on questions about individual accounts, citing privacy and security reasons. Users who are perma-banned from Twitter are, as a practice, not told which tweets of theirs prompted the ban - only that they are banned, that their accounts will not be restored, and which part of Twitter's rules the company says were violated.
Twitter also doesn't have publicly available guidelines for the threshold that must be met for a permanent ban; it only states in its rules against abusive behaviour that "any accounts and related accounts engaging in the activities specified below may be temporarily locked and/or subject to permanent suspension".
Twitter declined to provide the Washington Post with additional information on which policy violations have triggered permanent suspensions in the past, or the frequency with which Twitter hands them out for abuse and harassment violations.
Below are five high-profile instances of Twitter's permanent ban in action. But before we get to that, here's a caveat: While we've done our best to determine the status of each of these accounts, a permanent ban can look a lot like a temporary suspension from the outside. Some users who have been temporarily suspended from Twitter may simply choose not to take action to restore their accounts.
In three of these cases, the banned individuals have provided screenshots of the messages Twitter sent them announcing their permanent bans. In the other two cases, the context and fallout from their suspensions make it seem extremely likely that the bans were permanent.
Twitter permanently banned Yiannopoulos - aka @nero on Twitter - in the immediate fallout from a racist abuse campaign against Jones. Jones quit Twitter on Tuesday, after posting screenshots of an onslaught of comments she was getting.
Yiannopoulos often used his popular Twitter account to identify and mock enemies of the "alt-right", a grouping of anti-PC diehards, trolls and racist meme lords who have united around their common liberal targets.
On Tuesday, Yiannopoulos started making fun of Jones, particularly her response to the racist abuse she was getting. "EVERYONE GETS HATE MAIL FFS," one tweet read. Another called Jones "barely literate". Later, he shared faked screenshots that made it appear as if Jones were making profane and offensive postings.
Twitter didn't say exactly why it banned Yiannopoulos, only telling the Breitbart writer that he was permanently banned for a violation of their rules "prohibiting participating in or inciting targeted abuse of individuals". Yiannopoulos said the suspension was cowardly. Reactions to Yiannopoulos' suspension fell along the lines you'd expect: His supporters - he had more than 300,000 followers at the time of his suspension - rallied behind the #FreeMilo hashtag, which trended in the United States for several hours, and said that he'd done nothing wrong. Many others cheered Twitter's decision to ban someone whose mocking, trollish tweets about people on the alt-right's bad side were often the prelude to a mob of abuse.
The Yiannopoulos ban was part of Twitter's response to the abuse targeting Jones, Twitter said in a statement.
Yiannopoulos was subject to several warnings from the social network over the course of his Twitter career and had lost his blue verification checkmark in January for violating Twitter's rules. If he tries to return to Twitter, the site will likely notice and take action to remove him again, quickly. The question becomes whether it's even possible for Twitter to give the same scrutiny to the mob of regular accounts that harassed Jones on Tuesday. How long would it take for Twitter to discover if they return?
OnlineCensorship.org tracks instances of account suspensions and content takedowns across several social media platforms. Its data suggests that not all instances of rule violations play out the same over time and that one major factor seems to be celebrity. "Regular users, when they lose their account to a ban, can simply start up another and go unnoticed (unless they attract attention again or get reported)," said Jillian York, a co-founder of OnlineCensorship.org. "We see this with Isis-supporting users all the time - their large-follow account goes down, and the next thing you know, they're regaining on another one."
Charles C Johnson
In his time as Twitter's most notorious troll, Johnson did a lot to earn his reputation as an internet villain. He published the home addresses of New York Times reporters. He broadcast the full name of the woman at the centre of Rolling Stone's now-discredited University of Virginia rape story (and circulated an image that he said was of her - it wasn't).
Johnson, who is practising his particular version of bounty hunting-as-journalism elsewhere, seemed to always have a target, or a good sense of the line between accountability and malice. But it was a tweet asking for funds for "taking out" Black Lives Matter activist DeRay Mckesson that, it appears, prompted Twitter to ban him permanently from the platform in May 2015.
The ban came about a month after Twitter expanded the language it used to describe prohibited threatening behaviour on the platform.
Johnson had a clear history of violating Twitter's rules against abuse and harassment, even when the list of banned behaviours was much shorter than it is today. And many, including several of his targets, cheered his suspension as long overdue.
But some writers and anti-censorship advocates noted at the time that the particular way this suspension played out raises some questions about how Twitter enforces its policies. Over at Slate, Amanda Hess suggested that the tweet that finally did in Johnson's Twitter days may have drawn such a dramatic response because of the prominence of the person it targeted. Noting that a memo from former Twitter chief executive Dick Costolo once expressed concern about the effect rampant abuse was having on Twitter's "core users", Hess noted, "With 133,000 followers, DeRay Mckesson is one of them."
Zimmerman, best known as the man who was acquitted of criminal wrongdoing for shooting to death Florida teenager Trayvon Martin, had a second career as an online troll. After several major controversies about the content of his Twitter account, Twitter appeared to kick out Zimmerman for good in December, just after he posted a series of tweets with semi-nude photographs of his ex-girlfriend and her contact information.
Unlike Johnson, Zimmerman hasn't publicly stated that his ban was permanent, and Twitter doesn't confirm that sort of thing to journalists. But going on the fact that his account has been offline for several months, combined with the severity of the rule violations, it seems likely that Zimmerman was permanently suspended.
At the time, Twitter directed us to their policy banning "revenge porn" in response to questions about Zimmerman's ban. The now-deleted tweets accused Zimmerman's ex of sleeping with a "dirty Muslim" and of stealing from him. Twitter's revenge porn policy bans users from posting "non-public, personal phone numbers" of others, along with "non-public, personal email addresses" and "intimate photos or videos that were taken or distributed without the subject's consent". Zimmerman appeared to violate all three.
The tweets prompted speculation that Zimmerman might find himself in trouble with the law yet again in Florida, this time for violating the state's revenge porn law. But Mary Anne Franks, a law professor at the University of Miami and legislative and tech policy director of the Cyber Civil Rights Initiative, told us after Zimmerman's Twitter suspension that this was likely not the case. Basically, the photographs Zimmerman posted did not violate the state's narrow definition of "nudity". "It's possible," Franks said, "that Zimmerman may have researched the law to make sure that he didn't violate it."
While Zimmerman's account remains suspended, he has still been able to generate controversy on the social platform. His name trended on Twitter in May after he decided to auction off the gun he used to kill Martin.
Robert Stacy McCain
Twitter permanently suspended the alt-right, anti-feminist writer McCain in February. McCain has repeatedly questioned Twitter's explanation - sent to him in an email, which he posted to his blog - that he was suspended for inciting harassment.
His suspension became a rallying cry for those who believed a never-proven theory that Twitter was conspiring with feminist activists to censor conservatives. They started tweeting their outrage with the #freestacy hashtag, and then claimed that it, too, was being censored from Twitter's trending lists.
Twitter said McCain was suspended for participating in "targeted abuse", a rule that McCain sees as essentially meaningless. "Anybody could be offended by anything," McCain told me in a lengthy, wide-ranging phone conversation in February, shortly after his suspension. "The question is, who complained and why was it construed as targeting abuse?"
I asked McCain what he thought of Twitter's stated position that online harassment can have the effect of intimidating its victims into silence. He replied, "That's crazy."
McCain, whose account was locked in the past for rule violations, has said repeatedly that he was doing nothing on Twitter to deserve a suspension. And because it became a big deal after the fact (once the tweets were already deleted from Twitter, along with his entire account) it's impossible to go back and look at the record of what he said before his suspension. That, perhaps, is why this case so intensely captured the support and imagination of users who felt Twitter was taking a stand against them for their conservative beliefs.
When asked about the theory that Twitter was cracking down on conservative users in February, a representative categorically denied it, saying: "When abusive content is reported to us, we will suspend the account without regard for political ideology."
Banks had a long history of picking fights on Twitter before her indefinite suspension from the site in May - to the point that some argued that her trolling had overshadowed her career as a rapper. Her Twitter beefs are well-documented in the music press and are often characterised by homophobic slurs. Once she got into an extremely graphic and bizarre Twitter fight with former Alaska Governor Sarah Palin, after apparently misreading a "hoax" article about Palin's views on slavery and believing it was real.
But the tweets that appear to have led to her suspension targeted singer Zayn Malik and a 14-year-old Disney star named Skai Jackson. The fight began with Banks accusing Malik of plagiarising one of her music videos, and quickly escalated until Banks called Malik a "curry-scented b****". When Jackson tweeted that "Azealia Banks needs to simmer down a little", Banks replied that the teenager needed to "grow some hips and start [her] menses".
The exact action - indefinite or permanent suspension - that Twitter took against Banks after that isn't clear.
But her subsequent attempt to return to Twitter under a different handle and address the situation led to that account being suspended, too, suggesting that Twitter doesn't want the rapper to return at all.
Before her second account was suspended, Banks tweeted that she believed Twitter suspended her because they "have a problem with a black b**** speaking her mind but not Isis accounts, Nazis, child porn etc". She added: "Something's wrong when a black woman's opinion calls for suspension. Where's @realDonaldTrump suspension?"