Another way is to use merit as a currency )
For example, somebody posted good relevant messages and got merit.
Now, each time the user posts in a few subforums (for example "Bounties") one merit is deducted.

That way, people who really contribute to the forum can use their merit to post ads.
It may be against the original forum idea, but when bitcointalk was created nobody imagined it would be hardly spammed by signature bounty hunters.