Netstar consultant refused to cancel contract.

I had a contract with Netstar for my vehicle which I sold at the end of January. I called Netstar to cancel and spoke to Brandon who asked if the new owner was buying the vehicle together with the tracking device. When I informed him i was not removing the tracking device he suggested he sends me forms to give to the new owner to fill and send to Netstar. A couple of months later I got a call from Hybrid, their debt collectors, informing me that i owed Netstar and that i had not cancelled the contract. I find this to be in bad faith, when i spoke to Brandon, my sole intention was to cancel the contract because I had sold the car. Instead, he did not cancel and left the contract open and expected me to chase after the new owner to take up a contract with Netstar so he could cancel! That is NOT my responsibility, if Netstar wanted business from the new owner, surely they should not have expected me to get it for them. I did my part by calling and informing them i had sold my car and was cancelling my contract. Instead, I have been taken advantage of because of my ignorance of the cancellation procedure and my trust that Brandon would cancel the contract as I had intended. Today I am being threatened with blacklisting if i fail to pay, for a blunder committed by their own. I have been with Netstar for 6 years and this is the thanks i get.