Left MTN years ago... but MTN never left me.

I ported away from MTN years ago.

To-date I still sometimes receive calls/SMS'/notifications from MTN to my cellphone number.

Every time I have tried unsubscribing I receive messages like "this number is not available on this network", because (guess what) I'm not with MTN and thus cannot dial the MTN specific codes like *135#

How do I get my details removed from MTN's various databases so I can finally not be badgered by their various marketing strategies?