NETSTAR: Taken for a ride

I sold my car which had a tracker. I called Netstar to inform them that I sold the car and they should cancel and deactivate the tracking device. Mind you I have two cars under Netstar. The gentleman I spoke to told me that it's impossible to cancel until I firm the term contract which is three years. I mean it will be easy for me to keep track on the sold car and know every move they make. So to cancel it I have to pay for the three years remaining forgetting that I still have one car with them and in the process of buying another one. But with this bullying, all the other insurance did not charge me they terminated immediately and to top it up I have credit which they will pay back to me. I would like Netstar to terminate my vehicle with no strings attached. The car in question JGZ 841 MP