Cartrack poor setvice

We installed a tracking device more than 3years ago the first18 months it worked perfectly then the trouble started, they could not locate our vehicle any were then they send a technician to fit a new device, after it was done I was told I just drive around town to see if they locate my vehicle for he cannot pick it up by GPS, ones the said my vehicle can be located near a church, but the nearest church to our his is approximatly 5 kilos from us, iv Sept I cancelled my contract with them, but they are still billing me and also I must pay R600 for the device they out of there own installed in our vehicle, nou they threatened me with blacklisting and there attorney, wat must I do they also refuse to remove there device from my car