Another way to retire user accounts is by deleting them. This approach is only recommended for accounts that have not been active (i.e. haven't reported issues). Once the account is deleted, any issues or actions associated with such account, will be associated with user123 (where 123 is the code of the account that was deleted). Note that associated issues or actions are not deleted.
As far as the underlying database, after the deletion of a user, records with the user id as a foreign key will have a value that no longer exists in the users table. Hence, any tools that operate directly on the database must take this into consideration.
By default administrators are the only users who can delete user accounts. They can delete accounts by clicking Manage, Manage Users, locating the user to be deleted and opening it details page, then clicking on the "Delete User" button which deletes the user.
Note that "Deleting Users" is not a reversible process. Hence, if it is required to re-add the user account, it is not possible to recreate the user account so that it gets the same ID and hence retains its history. However, manually creating a record in the users table with the same id, can possibly do that. However, this approach is not recommended or supported.