I was talking to someone who participates in another forum with different software. That forum has a "Siberia" mode in which the Forum Idiots are not exactly banned. They can still post all they want. The catch is that they are the only ones who see their posts. The people in Civilization are no longer bothered by them. They happily exchange their idiocies amongst themselves and are only mildly annoyed that no sane person ever responds to their posts.
The VBulletin folks may want to look into this.
There's a similar feature in VBulletin, I think.