Hi @d1612k , welcome to the Auth0 community!
It’s not possible to completely disallow signups from an enabled social connection. 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.
Using Actions and Account linking, you could look to implement a custom action flow that checks whether the users email already exists and if so link the accounts or block the account if a matching email does not exist.
I recommend having a look at the info about account linking in more detail if that is the path you wish to take.
Hope this helps!