Impossible to cancel terrible service

I painfully endured a 2 year contract with cell c, when the contract was finally up, i got numerous calls to upgrade or renew the contract. I asked them to stop any further billing as i want nothing to do with cell c any longer. But the upgrade department is not able to cancel further billing apparently. I have tried the cell c shops, phone numbers etc repeatedly, but either they dont pick up or put the phone down or are not able to do anything. I'm absolutely over being debited and billed for a service that i do not need or want. I want this company out of my life as fast as possible and i dont want to be held prisoner because they make it impossible to cancel their services. Why do they make it impossible for people to leave ? when they cant deliver acceptable service.