They have community driven feedback mechanisms. When a user is flagged sufficiently often, they're pretty much gone. Of course this is now abused against (some

They have community driven feedback mechanisms. When a user is flagged sufficiently often, they're pretty much gone. Of course this is now abused against (some) X people and by (some) X people (for X being pretty much any cause or philosophy that creates uproar and with-us-or-against-us camps).