Harrasment by Cartrack

I sold my vehicle Saturday 29 April 2024. Monday 01 May 08h00 I was at cartrack middelburg to cancel the contract and told where the vehicle was to remove the unit. They did nothing untill Friday 05 May then advised me they processe the cancellation but have not retrieved the unit. THEY told me I owe them R2800 for the balance of the contract. I refused then I paid them 3 months notice and then received a letter stating the contract has been cancelled. They however keep harassing me to pay the balance whuch is now R2 300 which i still refuse because i dont own the vehicle anymore and have done all i could to do the right thing