Impossible to end a contract with MTN

I signed a 24 month contract with MTN for a sim card to be used in my iPad. The service was appalling with strikes, no service, etc. When I tried to cancel it way after the expiry of 24 months MTN will not cancel despite my calling at Umhlanga Rocks on 18/07/2017 16.40 ref ********** to record cancellation. I have a printout of the MTN System recording this. 30 days notice would be 18/08/2017. MTN continued to charge my Discovery credit card. I went to MTN Rosebank monthly with my proof documents. MTN continue to charge my Discovery Credit card. I changed my credit card with Discovery. Discovery advise MTN are still charging my old number despite an email advising them again of the cancellation details now 4 months on. I have spoken to Patience Dlamini, Tumelo, Thamanqa Madikane, etc. Staff are rude and clearly don't do anything to cancel a mere amount of R54.99 debited to my Discovery card. Changing my Discovery card at huge inconvenience has not helped. How do I get this debit cancelled? I might add that if I was given a free 24 unlimited contract with MTN I would not use it. MTN service is the worst I have ever experienced.