Karma, reputation or scoring system to encourage posting in certain forums?

[Cyburbia]

I have a growing problem with off-topic posting on my board (http://www.cyburbia.org/forums/index.php). The primary subject matter is urban planning, but about two-thirds of all posts are in an off-topic subforum. The prevalence of off-topic posts is making it difficult for the professional planning community to take the site seriously, even though it's been online since 1994.

I'm looking for a karma or scoring system that will encourage people to post in the on-topic subforums. Something like:

* A display of the number or percentage of posts in on-topic areas
* A karma system that gives users more points for posts in the on-topic subforums.

I've ruled out disabling post counts for the off-topic subforum. I'm afraid doing that would drive off some users.

Any hacks that do this?