Drupal - How do I turn off account creation?

The answer given on How do I remove the user registration/login form? is relevant for this question too.

While it's technically possible to remove the login functionality, you shouldn't, because then your administrator could never log onto the site.

In order to remove the register functionality, you simply need to browse to admin/user/settings (for Drupal 6) or admin/config/people/accounts (for Drupal 7 and 8) and select the the "Only site administrators can create new user accounts" option.