The easiest way to keep people except those logged in from seeing anything is to require people to login/register before they can even access the forum as a whole. To do this go to Users/List User Groups then click on Unregistered/Unconfirmed and make sure no is selected under view. Here is a screenshot of what you are looking for.
Like I said doing it this way guests and non logged in people can't see anything.
But then you lose all the SEO benefits because you're hiding it all. You can try using the Google first click free addon. That protects your stuff but lets Google inside to index your content and will help drive traffic to the site.