Refusal to cancel contract

I am being harassed by Cell C, every day, every half an hour fro morning until evening. Although I cancelled my contract a month after it had ended, Cell C refuses to cancel the contract and I am continuously billed for services I have not used. When I enquired from one of the calls, the person on the other end said, "you will get all your benefits when you have paid. Cell C will not cancel until you pay up!" For 7months I am being held hostage by Cell C.