I see a number of posts in Community, with various answers, e.g.
- Disable Social Signup..except for Linking existing account
-
Is there a difference between login & signup for social? - #4 by assaf.shlomi
I am sure you can find more as you know the space far better.
Linking of accounts seems to exist as well. albeit with separate accounts (there go the stats) and a very complicated setup.
For cases like ours, where social login should be allowed for the ease of the login process but only for previously registered users, it should be enough that a stranger clicks on social login button and gets an error unless an admin had created an username/password account on beforehand with the same e-mail address. Where do I have this list of users - in Auth0 database, that is in https://manage.auth0.com/dashboard/eu/vacilando/users