Is there a common method to password protect registration pages?
Well, till they register/login, you can't actually say who they are.

I will suggest a CAPTCHA in the registration form. If you want to restrict it more, then you can search for email white listing modules. https://drupal.org/sandbox/amrit_b/1768342 This is something I wrote few months back. This is still in dev. In case you don't find anything else, you can try this one.