An idea - maybe you could make it a rule that if you have signature text or avatar in place, that you get a limit to the number of posts you can do a week.

If you have no sig, then you can post as much as you like.

For example, if you are a member with a sig - you get ten posts. If full then 15.

It would prevent the spam campaigns that just encourage fifty posts a week of nothing (i'm looking at you gambling website campaigns!)