Unable to cancel an expired contract

One of my three contracts has come to an end. I can get through to retentions but it is impossible to get through to cancellations. Five emails since December 2020, with 5 different reference numbers - no response. I have now created a stop-order with my bank to reverse the last payment - no response. I called accounts department today who then tells me that I have failed the security questions and as a result cannot access my account. Clearly I don't know my own details, right? I am sorry Cell C but you cannot force me to keep an expired contract open. I am reporting you to Icasa today