We used to have this feature, back in the old dial-in-BBS days, on Citadel BBS. It was called "Coventry."
A user who was placed into Coventry could see his own messages, and the admins/moderators could, as well, but nobody else would see them. This was done by marking the user AND the messages with a "Coventry bit." That way, if the user posted something worthwhile, the "Coventry bit" that had been set for the message could be reset without affecting the rest of his messages. Thus, the worthwhile message WOULD become visible.
Most of the time, such users would get increasingly worse for a short period of time, then never come back again. In some cases, though, they realised that nobody would respond to their rants - only their helpful messages, and they would quiet down to become good, productive members of the board.
Needless to say, as a former sysop of a Citadel board, I would wholeheartedly encourage the development of such a "ghost posting" mod for SMF, and would be more than happy to help test it out.