I am using the registration plugin. My website will be in private beta so I check the user email address against a white list.
How can I tell facebook that the registration failed?
Also, How do I unlink a user? Since it failed, the user can’t re-register right now because facebook thinks he’s already a member.
The registration plugin includes several advanced features like being able to validate the form data. Using this you could check to see if the email address is in your whitelist before the user actually connects
https://developers.facebook.com/docs/plugins/registration/advanced/#async