Login with Google, how do I restrict it from making new accounts?

I have my application to only log in with Google by using the workflow sign up / login with social media. How do I prevent it from making users and only allowing existing to log in? I didn’t even realize we had this huge security hole here until my one user today let me know that she accidentally picked her personal gmail account and was using the application with that, which messed up a ton of records since it didn’t have any of her user info. No user should be able to sign up for our application but instead need to be created by myself and then linked to their gmail later. How do I do this?