If you have a set group of people that need access, make a new user group in your control panel and add those people to it. In the "edit forum settings" link, give access to that forum to only that group.
We did something similar here with a forum that only guests can see... log out and check it out I even have it turned off to admins so I don't have to trip over it when visiting while logged in
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.
I even have it turned off to admins so I don't have to trip over it when visiting while logged in 
