Disabling unauthenticated sign-ups

If you're using built-in authentication, you can block unauthenticated people from being able to create an account.

From an administrative account on GitHub Enterprise Server, click in the upper-right corner of any page.
In the left sidebar, click Management Console.
In the left sidebar, click Privacy.
Unselect Enable sign-up.
Under the left sidebar, click Save settings.
Wait for the configuration run to complete.