Please somebody help me to cancell my contract with Cell C

My contract with the Cell C company expired and the keep on calling me forn an upgrade but it never happen because every i operator that calls me tells me me that the need to send a request to their credit department, what surprise me is i asksed the m to cancell the contract but the keep on saying they wii contact me back and the still debit my account every month and i dont wanna reverse the debit order because they are good in listing client on ITC.