MyBB Community Forums

Okay so my forum is growing pretty fast now and I'm getting about 10-15 members every couple of hours but the problem is that nobody is posting so I want to be able to only allow people who have made one or more posts to access certain forums (but still be allowed to access other ones) but also I want a message to be displayed to the user giving them a reason why they cannot access some parts of the website.

Any help is appreciated, thanks!