You have to run the supervisors outlook with the cleanrules switch once . Then the NDR should disappear.
Did you check the delegates tab in tools, options? If the name or SID is in there, remove it. Close outlook and re-open to confirm it is gone. If it comes back then you need to remove the name from delivery options on the exchange general tab in Active Directory on the Supervisor’s account. Then go back into the delegates tab and remove the name again, this time it should stay gone. We get this issue often, it’s why I tell people not to use delegates in our organization, but still many do it anyway. Such is life.