Ha! Tried that. I set up a forum where new registrants could post, and prevented them from seeing any other forum even.
In an ideal world this would work, but what about search engine traffic? If SE's can't see data they can't index it, so you won't get much traffic to your site by way of them.
It was the other way around: regular users and not-logged-in visitors could see all the usual forums. But for newly registered users, all they could see was that one forum. Crawlers saw all but the one forum, as well.
The spammers never noticed the odd setup, and just posted away. I got tired of watching it, and blew them (and the new forum) all away.
It was interesting, though. Some of the spammers came back and posted repeatedly, while others never came back and never posted.
