MTN continues to debit for cancelled contract

I had a contract with MTN, which ran its course and which I cancelled in writing, giving 30 days notice as requested. I received confirmation of the cancellation in writing. BUT they continue to debit me with the subscription amount. I stopped the debit at the bank, but they have changed the amount so it goes through (bank stops are flagged on amount). I get absolutely nothing from MTN except a computer generated response. Plenty of talk and no action!