Guests can't enter certain forums

[lilah]

no reason to feel sorry, I know it's complicated

basically i would like to have 2 kinds of forums:
(a) users can see the forum on the index, they can enter it, they can enter the threads and posts replies
(b) user can see the forum on the index, they can not enter it, they can not enter any thread in it and not post replies

i would love it, if they got the "You are either not logged in or have not permission to view this page. ..."-error, when a guest clicks on a (b)-forum