Can I prevent Facebook Ultimate allowing new users

I have setup (I think) Ultimate Facebook and want to allow admins to log in via Facebook, but want anyone else who goes to the admin login and tries to connect to be rejected.

Eg: Only allow existing members to log in.

Should this work or will it allow new members to `sign up'.

Under the section Facebook Connect I have:
Allow users to register with Facebook: checked
and all other options as default.