Debit order termination with mighty MTN

I have been struggling to cancel my contract with MTN since last year September. I paid off my account they no longer credit me with minutes nor data HOWEVER they still deduct R420. I have called them, emailed even spoke to the lady by the name of Mapitso. She promised to cancel the debit order but still today that has not happened. Is MTN so poor that it cannot do WITHOUT my mere R420??? Let me make this know I will reverse it until they get it through their thick skull that I do not require their services anymore.