Harrasment on closed account

More than 5 years ago I stopped working and I contacted Cartrack to closed/cancel my contract with them as I was going to sell my car, I was assured that it had been cancelled. My payments were up to date at the time so I owed them no money at all. Early this year I received a call saying that I owed them over 5000 rand, I explained to them that the contract was cancelled over 5 years ago, the person on the phone said \oh okay"and hung up. Ever since then I receive these calls every 2-3 months"