Contract cancelled but still after four years they harass me

I was for a presentation at Zone Fitness in Willowbridge three years ago. They informed me that the contract is only for a year. After a week I cancelled the contract for medical reasons. They still debited my account and I stopped it at the bank. After two years they started to harass me with several laywers and debt collectors. I informed them that I have cancelled the contract and I never ever once set foot in the gym. Now after four years they have sold the debt to Norman Bissett and Associates and is threating to blacklist me. They help me responsible for an amount of R2815.26. As this is old debt and older than three years this is prescribed debt. I never ever received any correspondence regarding this matter in the past three years and did not make any payment.