Beehive Forums has this feature. It has a "worm" mode function where if a user is flagged with that, they can see all their posts... but nobody else can. It is similar to Reddit's shadowban.
It works well, because instead of the troll creating yet another sock from a different IP, they will spew their nonsense all over the place, and nobody will be the wiser.
I've wanted a feature where an admin could post with a flag where only admins and the troll could see the reply, mainly to keep the troll interested in keeping the account that nobody else sees. Perhaps an ELIZA-esque bot can reply to the troll (again, where only the troll would see the reply) which keeps them busy.