Closed account still showing as active

I had a contract with cell c that ended almost a year ago. This contract still shows as active on my credit report and shows that I have to pay R400 on this account. I have asked cell c to update their records so many times, they always they will, they even sent me a paid up letter but failed to update my credit information. I don't know what else must be done for them to update my details to show that the account is paid up and closed.