Cartrack is harassing me!

I am being harassed by Cartrack a year after canceling my contract. I sent my cancelation form and paid all that was due. A year later they have started debiting my account randomly and insisting I owe them. I have asked countless times for someone to explain where this money i am supposedly owing them comes from and no one has an answer but they expect me to pay it otherwise they will blacklist me. Cartrack is a big ****.