Failure to act on my cancelation letter

I had contract with Telkom for 24 months and it expired. It continued on a monthly basis until last year when I wrote a letter to cancel it. But to surprise Telkom continued to take money from my account. I then decided to reverse the payments. Now telkom demands R2400 from me for the arrears that accumulated after the contract was canceled. Still they do not want to cancel the contract. They threaten to blacklist me. Please help me. I want the contract canceled. I am a pensioner, i cannot afford to continue with the contract.