Sick and tired

Cartrack has been harassing me for months over payments that are not valid. I had a contract with Cartrack, and everything was fine until I wanted to cancel it. They then told me I had to pay a cancellation fee because my contract was for three years. I decided not to cancel and instead waited for the three years to end. After the contract ended, they told me I was now on a month-to-month agreement. I asked them to terminate the month-to-month contract, but they kept transferring me from one call to another without actually cancelling it. Eventually, when I managed to cancel, they started claiming that I owed them money—which I don’t understand. I don’t even drive that car anymore, yet they still call me up to 10 times a day, saying the same thing: that I must pay them. Yesterday, I told them I will not pay a cent. They can do whatever they want.