MyBB Community Forums

This will be a great way to avoid spam through PM.
Users get an error message they cannot send pm's before having X amount of posts when trying to send one.

It's an idea of a IPB plugin:

You can use Group Promotion for this, although it will not show that message.
Disable the private system for registered users and create a new group "Verified users" (or something) and enable the private system for them. Now set up group promotion to change the usergroup of registered users with more than 30 posts to the new "verified users" usergroup.