Cannot delete; user &1 is in use as a reference user

What causes this issue?

You tried to delete a user that is used as a reference user for other
users.

System Response

The user was <ZU>not</> deleted.

How to fix this error?

Delete all references to this user before deleting the user.
You can use transaction SU10 to do this as follows:
Start transaction SU10.
Choose the 'Authorization Data' pushbutton.
Enter the user to be deleted (&V1&) in the "Reference User" field.
Start the report.
In the result list, select all users and choose the "Transfer" button to
transfer all affected users to the worklist in transaction SU10.
You can now change the users and assign a new reference user on the
"Roles" tab page, or remove the previous reference user by leaving the
field "Reference user for additional rights" empty.
Select the "Change" checkbox and save your change.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.