When a user is deleted, he’s not removed from the database because the object log and the syslog has recorded the user’s events. Instead the user is flagged as deleted in the database.
Deleting a User
To delete a user, open the user and click the Delete button.
Deleted users cannot log in. If a deleted user attempts to log in, the attempt is logged in the SysLog.If the user is currently using the database, they are immediately logged out.
Viewing Deleted Users
A deleted user is still in the database, but is flagged as deleted and does not show up on the normal Browse Users tab.
Deleted users don’t appear in the normal filter setting in the Browse Users tab. To view the deleted users, click the “Deleted” checkbox.
UnDeleting a User
To undelete the deleted user, open the user and click the Reset button.
When the user was deleted, all team memberships and any admin rights were removed. Once the user is reset, you'll have to re-assign any needed permissions.