Wasn't there a hack that made selected users believe that their posts displayed normally but actually it didn't show to other users? *Scratches head* You can always set Admin-allowed registrations only. Anyway, sometimes simply ignoring an unwanted user works.
thats a option in vbulletin already. its under banning options called "
Tachy Goes to Coventry" you just put the userid in the field and no1 can see their post.