For a private site where no forum content is visible to guests, the easy way to hide everything is to simply deny "View" permission to Unregistered / Unconfirmed users.
However, the end result is a little messy as guest visitors get presented by an unhelpful error message "You must be logged-in to do that" and a login form, and the page returns a 403 (Forbidden) error.
This simple addon provides an alternative approach with a cleaner and more elegant outcome.
The following UI...
