Cartrack *******

I am being harrased by Cartrack, I finished my three year contract with them and called them wanting to cancel and was told to serve 20 days notice, I did request cancellation via email, served 20 days and then they just kept wanting to debit my account, harassing me with calls, after I explained, then in September I was asked to complete a cancellation form which I had never been told about when I cancelled in July, this was just to attempt to ***** money from me. They have threatened me with lawyers