Got this idea from my old site.

Subforums.

Boy's Privacy.
Girl's Privacy.
VIP Forum.

Boy's Privacy:

Males with a set ammount of posts (300 or so) are given permissions to view a forum only for males. Anything can be posted (My old forum allowed nudity and such).

Girl's Privacy:

Same as males.

VIP Forum:

The original joiners of this forum are placed in a User Group for VIP's and have access to their own forums. Every so often new members can be chosen to be part of the VIP group. Basically a way to reward the long term members for being here.

Another idea, if the point mod is installed, is create a forum you have to purchase to be in. Just another thought, not a very good one though.