Setting up Facebook-integration in administration you can disable user creation, only allowing existing forum users to login with Facebook. But when the user registers both options is still presented to them (with only one working).

Should the option to register a new forum account not be removed when the administrator has diabled it in the settings?

As far as I can see it is pretty simple to do, and can fix it and make a pull request if you want.

(The same might be true for other social media integration, I have not checked, but the logic should be the same)