A bit of a reverse method:
In the Administrative Control Panel, click on the forum/subforum you want to make private in the left-hand box. On the right-hand side, click on the Permissions tab. A list of users should appear; click on "Create" next to a username that you want to PREVENT from accessing your forum/subforum. "Create" should turn into "Edit / Delete" so click on "Edit" and you'll see a list of permissions. Set everything to "NO," save, and that user should no longer have access to your private forum/subforum.