ok, fine, make a passsword protected forum and don't add it to a category <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" /> ... Or set it up with any of the private forum permissions and don't add it to a category; only thing is your members will need to know the forum number to enter it since it wont be on the list...
Not to inundate you with suggestions, but if you're interested, UBB.threads does do permission-based hidden forums. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />