The miniOrange SAML module automatically creates a user on a first-time SSO login if the user is not present in Drupal and updates them on further logins. If you disable the auto-creation of users then users can log in through SSO only if it is present in Drupal as well as IdP. This will ensure that no new user can log in through SSO login. (Note: This feature does not disable the user creation from the Drupal login form)
Steps to disable auto creation of the user:
Navigate to the Signin tab of the module
Check the option - Disable auto creation of users if user does not exist.
Scroll down and click on the Save Configuration button.