CSDA1 you can disable registrations under the permissions tab.
There is however no way to disable password login natively. If there's no password accounts in the database, you could simply hide the password login fields with CSS as no one will need them anyway.
The only potential issue is that the "reset password" endpoint still works for all users in the database, even if they previously created their account via a social login.
Some extensions like my paid Wordpress integration do include code to completely disable password login, however it's linked to whether the user is synced, so I can't easily split that code into a standalone extension.