Restricting guests from one forum.

Okay... not sure what the problem is, but I run www.subnixus.com/forums and I have a new section of the forum for adults only. For legal reasons people MUST be registered and login to view this part of the forum. However, when I uncheck all of the "guest" boxes in the permission area for that part of the forum, guests are still allowed to enter, view and post. What am I doing wrong here?