Car Track refusing to end contract on a written off vehicle

I have car track on my vehicle which is less than 6 months old, the vehicle was in an accident and has been written off. Now Cartrack is insisting that I should pay the monthly instalments even though the vehicle in no longer in use and has legally been written off. How is this supposed to be legal and to the benefit of the customers, its not a planned event or moving to another company. Its an accident.