When you have a setting like this:

Can View: Yes
Can Read: No

Set for as an example Unregistered Guests. It does not ask them to log in or anything.. they just click the link and get a blank forum instead of a notice or anything.

There should be a login, or a notice saying they do not have the proper access to that forum.

Basically.. if they aren't logged in it should ask them to, and then it should see if they should have access to that forum.

http://area51.phpbb.com/phpBB/viewtopic.php?f=3&t=25848" target="_blank

I do this for unregistered guests so they can see that a forum exists, but so they cannot read or post without logging in.