In the admin panel -> Permissions, you can set 'Read permissions' as 'Members only'.
If set, when a new user comes to the board, they dont see anything, but a blank discussion layout with the 'start new discussion' button. This is a poor user experience, so there should really be a landing page for an overview and say to log in or something.
I've had a dabble with the basic plugin stuff to adding things to the UI, and logging in using x service, but how would you flip a template based on if logged in or not?