I've seen this worked on a UK website I am a member of. In order to even post in the forum, you have to meet a certain requirement (X posts, X months of membership). My suggestion would to have a minimum of 500 posts and 6 months of membership to create a thread, 300 posts with 4 months of membership to post in a thread. There should also be a way to monitor the activity, so that someone can't create a profile and then just wait a couple months. I know that this is in no way a fool proof idea, but I thought it was a great little way to keep that forum a bit cleaner.
Cheers