Cancelation of my Cell C contract

I've been trying to cancel my contract with Cell C for a year now.

I had a contract with Cell C. I called them a month before the end date of my contract with them, informing them not to renew my contract.

They kept on deducting money from my account. I called them several times asking them to stop deducting money from my account. I even reversed the payment, but the following Month they deducted again.

I don't know what is it that one should do to make them to stop deducting money from my account.

It is so clear that I'm payment for something that I don't even know. Somebody is benefiting from my hard work.

I don't know what to do anymore. Please help.