Forums can be restricted at the following 5 "views":

1. Who can view the forums?
2. Who can view the thread listing of each forums?
3. Who can view the actual thread posts?
4. Who can post new threads?
5. Who can reply to posts?

For each view, you can specify which of your membership levels can actually participate.

For example:
1,2: everyone (even non-members).
3,4,5: Current members only.

...this way, you "tease" non-members but allow only members to actually read and post.

The way you actually do this is to let us know via Support Tracker, and we then set your config file accordingly.