Billing after contract has been cancelled

My cellphone contract expired end July where after I cancelled the contract and asked to be put on prepaid. This was confirmed by the consultant. MTN kept on billing me after numerous phone calls to the call centre. I have refused to pay anything further where after they suspended the line and now my sim card is useless. I have gone into 2 seperate branches where they have advised there is nothing they can do and kept referring me to the call centre. The call centre kept referring me to the legal department and so it has been going back and forth since July. One of the consultants at the branch actually showed me a pile of contracts that need to be cancelled and have been sitting on their desk for months. Also a manager was supposed to call me to sort this out and I have never received any call. This is such poor service and no one seems to be able to help me with this issue. I really just don't know who to speak to or who can assist me as I have spoken to so many people at the call centre with absolutely no result. I do not think it's fair that I pay when I have cancelled the contract when it came to the end of the period.