How to prevent users from creating accounts in Drupal 8
By default, Drupal 8 allows users to create an account but an admin has to approve it. An admin gets an Account Detail email with pending approval.
To remove this ability, go to “Configuration -> Account Settings -> Settings tab”…
Then disable registration by setting “Who can register accounts?” to “Administrators only” …
Remember to click “Save Configuration” at the bottom.
Now the login page no longer have the “Create new account” tab. And if they try to go to /user/register/ they will get an “Access denied” message (unless they are logged in as admin).