account cancelled yet instalment outstanding

I moved out of my premises 2 and a half years ago. I cancelled my Dstv subscription after contacting the accounts department. The consultant advised my cancellation had been processed. However, it has now been brought to my attention that R400 is outstanding against my name, 2 years later. This is ridiculous. This has now reflected on my credit rating,which has been excellent for the15 years. Dstv did not bother to credit the charge after me contacting them more than once. Please resolve multichoice. All people are not dishonest.