An administrator may "turn off" or suspend a user's account by selecting "No Login" in the "Choose an authentication method setting" in the users profile in edit mode.
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.
Note: An alternative way of suspending a user's account (available in Moodle 2.2.1 onwards) is to tick the 'Suspended account' checkbox on the edit profile page.
- Using Moodle What is the difference between suspending and soft-deleting a user? forum discussion