On a Vbulletin forum I used to admin we created a forum and user group called Purgatory.When we found an account like the shill one we moved them into that used group. They could still log in, see all the normal public posts but could only post in the Purgatory forum. No one could see their posts except themselves and the mods. Other members of Purgatory couldn't see them nor the general public. They couldn't send or receive PM's to anyone except mods. It was pretty funny watching their threads and replying to them occasionally. Eventually they got the idea and left the site. You might consider something like that here. It's not hard to setup.