Stealth banning (also called shadow banning, ghost banning or comment ghosting) is the act of blocking a user from an online community such that the user does not realize that they have been banned.
By making a problem user's contributions invisible or less prominent to other members of the service, the hope is that in the absence of reactions to their comments, the problematic user will become bored or frustrated and leave the site. If the user never becomes aware that they were banned, it will not occur to them to attempt to circumvent that ban.
History
Michael Pryor of Fog Creek Software described stealth banning for online forums in 2006, saying how such a system was in place in the project management system FogBugz, "to solve the problem of how do you get the person to go away and leave you alone". As well as preventing problem users from engaging in flame wars, the system also discouraged spammers, who if they returned to the site would be under the false impression that their spam was still in place. The Verge describes it as "one of the oldest moderation tricks in the book", noting that early versions of vBulletin had a global ignore list known as "Tachy goes to Coventry".
A 2012 update to Hacker News introduced a system of "hellbanning" for spamming and abusive behavior. Craigslist has also been known to "ghost" a user's individual ads; and reportedly entire accounts. Reportedly, an ad is placed and confirmation is sent that it has been posted; the ad may be viewed in the user's account, but, if ghosted, will fail to show up in the live listings.
In 2017 Twitter implemented a similar "timeout" feature where accounts could be temporarily restricted such that only their followers could read their message, although this restriction was announced to the user.