Read Shadow banning (Wikipedia)

Shadow banning is the act of blocking or partially blocking a user or their content from an online community such that it will not be readily apparent to the user that they have been banned. For instance, shadow banned comments posted to a blog or media site will not be visible to other persons accessing that site from their computers. By partly concealing, or making a user's contributions invisible or less prominent to other members of the service, the hope may be that in the absence of reactions to their comments, the problematic or otherwise out-of-favour user will become bored or frustrated and leave the site, and that spammers and trolls will not create new accounts.