Harassment and pathetic service

I have been harassed by Cartrack. Kindly be advised that the amount I owed was paid already which was the last debit order they took from my account. I did contact cartrack to cancel my account /contract therefore the amount they referring to falls after I had canceled the contract. Please also note that my 36 months contract we agreed upon expired long time ago, this was now month to month contract and was advised by cartrack that if I did not pay they just cut the service. I do not owe cartrack any amount and therefore please stop harassing me. Your services are even pathetic, you only call when you think someone owes you.