Unauthorised debit orders after cancellation

I cancelled my car tracker in January through phoning their call centre and it was confirmed that it has been cancelled. now everymonth I get harassed by them calling me telling me that I owe them, I called them and their rude agent tells me that they only do cancellation via email and I must pay before I cancel, pay for what when I don't even have that car anymore? I was told it was cancelled when I called in January now I must pay someone's incopetence. I will never use them even again and won't recommend them