Want to prevent adding new users

First a little background, I am new to Joomla, but have some php experience. I have setup a small site for a bunch of friends. I don't want to open up registration, so I am managing users manually. When I click on a social login button, and it hasn't been associated with a new account, it creates a new account for that user. Is there a way to prevent this behaviour, and only allow the user to login via the social buttons, but if the account doesn't exist, just redirect back to the login page? Ideally, this would be a configuration issue either through the SCLogin extension, of via a Core Joomla feature, but a indication where in the php code to modify, would also be acceptable.

Thanks for any assistance you can provide.