How do I restrict new users from using private messages?

[achtungbaby]

Sorry for the number of questions today...!

I want to be able to allow private messaging to members only after they've made, say, 20 posts. How can I do this? I currently have a promotion set up that kicks in after they hit 1000 posts, but the one I'm thinking of now is sort of a "pre-member" user group.

[tubedogg]

Create another usergroup, make it able to use PMs, then set a promotion to have people moved to that group after 20 posts. Set the 'normal' registered group to not be allowed to use PMs.

[Floris]

Keep in mind that the current vbulletin scheduler is limited to only update per hour, and not per x minutes. So users with x posts have to wait at max an hour before they are updated after their last activity to this new usergroup by the scheduler.