Protecting a Post of any type (e.g., Post, Page, Product, bbPress Forum, Topic, Reply) will protect the permalink leading to that Post. It will also protect any child Posts in a hierarchy. For instance, protecting a parent Page also protects any child Pages, and protecting a bbPress Forum also protects all Topics/Replies in that Forum. This works for any type of Post in WordPress, including Custom Post Types.
Here you will find a meta box that allows you to protect any kind of Post in WordPress. A bbPress Forum is in fact a Post Type in WordPress, so bbPress Forums will show up in this list. Simply choose which bbPress Forum(s) should be protected by the Restriction you're creating and it will then be off-limits to the public.