MTN South Africa cancellation

My contract ended May, i was INSTRUCTED by MTN that i should call on the 11th of May for cancellation, as I did. I did phone on the 11th of May and they confirmed that the contract was cancelled. I received an refrence number. "Foolishly" believing it was finilised. This month I received my monthly data and airtime, AGAIN! I contacted MTN and the first call the line just cut of, but after the second call, I was helped,but all they can do for me is to say sorry,but I have to PAY AGAIN this month for their mistake! I will NEVER use MTN as a network again. Hopefully this is my last month for payment, but I am going on maternity leave in 2 months and now the money I could have saved must go to MTN for a mistake on their side!