No go area. I am still struggling to get the faulty device to be removed from my vehicle

I installed a tracking device from them in January and few days after the installation I was always getting a call that they received a signal/panic from the vehicle, and I will tell them that the vehicle is fine until we realise that the device is faulty and I started to experience a power cut on the vehicle. I took in the vehicle for a repair as I was not getting any help from this company until today and they only sent me a cancellation form only today after I stopped their debit order and they still want me to pay cancellation fee after 5 months of hustle to get them to remove the device. The guy that was installing the device even said that he struggled to get the device installed but he is done. Little did I know that he was only working to finish not to do the proper job. I'm very disappointed in this company.