Wont cancel my service

I have decided to cancel my service with Cartrack after being with them for a couple of years. After emailing them in writing they refuse to cancel the service insisting that I phone and speak to someone. I think it is absolute nonsense that I have to jump through hoops just to cancel.

Now on top of it, the fees have also accumulated due to them not canceling and they keep harassing me with phone calls on a daily basis!