Been through this once before I believe with Don.
Yes setting guests access to zero will stop access to forum topics but will not completely close the board to guests.
They will still see the forum summary,portal page,etc.

This was a feature in classic but not today in threads.

Only way I see right now is to password protect the install directory. But that would be a headache.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers