An administrator may "turn off" or suspend a user's account by selecting "No Login" in the "Choose an authentication method setting" in the user's profile in edit mode, or by ticking the 'Suspended account' checkbox.
Different users have to have unique email addresses in their profile, even when email is not enabled. When the "No Login" setting is used, the account email may not be re-used to create another account. This essentially is a way to bar a particular user account from using Moodle for the duration that the account is assigned to this authentication type.
- Using Moodle What is the difference between suspending and soft-deleting a user? forum discussion