Unhappy customer

I regret the day I took a contract with cell c , I have been calling from March cancelling the data contract that im not even using, the last agent I spoke promised they will even refund me because he can see im using this sim and there's some stupid subscription that I didn't sign up for .when we speak on the phone we hear and understand each other ,but they keep coming to my account and messing up my credit score .what does it take to cancel this contract.