Prepaid but still paying for cancelled contract

My contract expired in July this year, and I cancelled it end of July. I received no reference number as the lady at Cradlestone mall branch said they can only submit the documents on the Monday. The next month, it wasn't cancelled and MTN deducted another payment from my account in September. I then requested that they cancel it, which they did, and moved me to prepaid. This morning MTN took another payment from my account, and my account status is prepaid. Please explain why MTN is steeling their own customers money???? I am really disgusted by their lack of service and bridge of contracts.