Hi @jesse2,
Unfortunately the Pre User Registration Action can only be used with Database and Passwordless connections, as detailed here in our docs
You could still use Actions to block users from a social connection based on email domain but this would need to be built as a Post Login Action. So the user would first authenticate with the social provider but on returning to your application you can disallow access to your app. Here is an example of an action used to deny access based on email domain.
I hope this info help, thanks!