Cartrack taking customer for a ride

I had a tracking device with cartrack for 1year ,3months and I sold my car and bought a new one. For 10days I have been asking cartrack to remove my tracker from the old car to the new car since my device is a contract one,but they failed to deliver for the past days. Now I asked them to cancellation the contract due to the failure to deliver on the part. Then they are now bombarding me with threat that I must pay them cancellation fee whereas I cancelled due to the none performance on their part.