Poor service and selfishly

I am so frustrated now, I had a contract with cell c for 4 years now because I have renewed the contract after the first 2 years and the second contract was ending in October which is the 4th year. So what happened I never defaulted any payment and in the month of September and October I have financial crisis and I called them to make an arrangement and to cancel contract as is ending end of October , however all the time they refer me to retentions and they always say they will call me to finalize cancellation but they keep on billing me and going to my account whenever they feel like but I made arrangement with them. Now my Bank account is on negative balance of over R600 because when ever they try to debit the bank charge me R147 for unpaid Debit order.