HELP ME DEAL WITH NETSTAR

I took a tracking device from Netstar from 2010 then on 2019 they called me to upgrade the the track of which I did ,on February this year i decided to cancel the track because i sold the vehicle ,the consultant said they will investigate then cancel the track but since then they ignored me from March to May the did not deduct and i have stopped the debit order . On July they called to inform me that i have an out standing fee I owe them ,when i tried to explain to them that i canceled the track on February with them they said they already took the matter to the legal representatives and they will blacklist me and they did not even inform me about the settlement even if i ask for it but they keep on texting me that I owe them and they will take further actions if i don't pay . The person I sold the vehicle to does not want the tracking device of which that was the main reason i had to cancel the tracker.please help me deal with Netstar because I have been trying but the is no moving forward I even took the vehicle to them to remove their device but they didn"t please help me i dont know what to do now.