Tracker Horror

I'm so disappointed and stressed by Tracker, my car had a problem and I had to trade it in this year on August, I called Tracker asking them to disable the tracker from the car I have traded in, but they refused and told me I should pay R1700 to cancel the contract. But to my surprise I don't recall signing any contract that stipulates the cancelation issue. I just traded in the car and I was Just expecting Tracker to disable the tracking device from the old car. Please assist, because now I'm receiving invoices I can't afford to pay for a car that's no longer with me.