Restrict to only FB registrations

I wonder is there a way of doing this, I want it so that the only way to register onto a website is using Facebook Connect. This will mean that users who already have a Drupal account should still be able to login but new users should not be able to access the register page without going through FBconnect first. Has anyone any ideas if this is possible and if so what combination of permissions or modules could be used to achieve it.