bbPress Access is a simple but effective add-on for bbPress powered WordPress sites. Using this plugin you can restrict access to your bbPress pages for group of users or based on login check.
This plugin will redirect them to a custom page that you create, if the user is not permitted to view the forum. You can create a custom page and set that.
Restrictions will be available on all bbPress pages and that can be done based on different roles, login conditions.