Just another idea:

What if new mebmes on the forum were disallowed to create new threads?
Only members with at least 50 or so posts or with 20 or so thanks's could post new threads.

While this would not be a problem for people who want to be part of the community, it would force new members to search for other threads to post their problems, and therefore they wouldn't be able to spam the "active threads" section.