Yes, simply set guests "view" level in each forum to "none" (permissions tab in the edit forum page in the control panel), then when they view your forum, they'll see nothing but a text similar to "There are no forums for you to view" (which can be edited in the language editor).
Views are cached; trust me, it's better like this... The cached data should be added to the amount of views every time there is a post anywhere on your forum though.
