Hi @michael.wise , welcome to the Auth0 Community!
Unfortunately, the pre-registration action does not work for Social Connections. This is because it is only when the user returns back from Authenticating with the social provider that auth0 knows who this user is. The blocking of the user is the only solution offered at the moment.
We have further guidance on this FAQ post,Remove social sign ups from universal login but still allow logins. please take a look for more information.