MTN Customer service poor

I contacted MTN to cancel one of my SIM cards because I had misplaced it, but they refused because I owed money on that contract. After paying it off, I requested that they cancel the SIM card. Two months later, the same card is charged a total of 2,000 Rand. I've sent many emails with no response. When I call, I am transferred from department to department, and nobody accepts responsibility. This is my last hope.