Cartrack refusing to cancel my contract with immediate effect

I am struggling to cancel a contract that I have with Cartrack with immediate effect. One of my vehicle has matured last month and the unit is no longer working and have asked Cartrack to cancel this contract with immediate effect. Unfortunately Cartrack said they can only cancel my contract with 20 days notice. I have tried to explain to Cartrack that the unit is not working and also received confirmation from Cartrack control room that my unit is no longer working and Cartrack client service is refusing to cancel my contract with immediate effect.