One solution could be:
- move all your forums to a separate course (you can use the backup-restore function to do this).
- Change the settings for this course: no guest login, perhaps you can even set an enrollment key and mail it only to students who need access..
- place a direct link to this forum on your site's homepage.
I hope an example makes things clearer.
Right now you are reading a forum in the course "Using Moodle". If you click on this link: http://moodle.org/mod/forum/view.php?id=1095,
you will directly be taken to a forum in another Moodle course (The documentation project course)
If you have never visited this course, you will be asked if you want to enroll. Of course, here at moodle.org there is no enrollment key to enter, but I hope you get the idea...