Cartrack - issues with having an old account cleared of debt that is not owed.

Our family car had cartrack in. When I traded the car in, I arranged to have the unit moved with cartrack. A few minutes later the dealership informed me there already was a unit in the car. I immediately emailed the lady to cancel the move of the unit as well as give notice. I received a confirmation email too, which I had sent a couple of times to Cartrack. Now Cartrack is pestering me for the money for the move of the unit. After answering call after call, explaining what happened, the one lady said she could see the unit was never moved and account closed, and it will be credited and rectified. I requested an email with this confirmation of, and still have not received it. One week later I still receive calls for the money of the move of the unit. I've sent another email explaining the whole story, and still I get these calls. I am fed-up with trying to get them to credit the move of the unit, as it never happened.