Trying to cancel contract for months

I have been trying to cancel a cellular contract with MTN for months now. Every time I go to one of these stores the sales person tells me that the contract will be cancelled and the number will be changed to prepaid and every month I receive an SMS stating that my account has been credited with airtime again. The sales persons have told me on more than one occasion that the contract will be cancelled and that I would be reimbursed for the months that it has taken them to cancel this contract. I have not received any payments from MTN nor have this contract been cancelled. The service is pathetic. How difficult could it possibly be to cancel a contract? I expect someone to contact me and tell me when exactly this contract will be cancelled. And when and how much money they will be paying me back.