To clarify on what Gizmo said - there is no way to hide forums from users that can not reach them in UBB.classic. This is due to our page caching system.
You can, however, hide forums in a number of ways. As of the 6.7 release, you can even hide categories.
That is probably your best choice - hide the category that you do not wish to be visible. You can still access that category and all the forums within it, but they will not be visible to users that don't know the exact URLs to each. (Do note that the URLs are pretty guessable, so moderately advanced and curious users will be able to discover that the forums exist.)
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