Cartrack refuse to cancel my contract

Hi.i desperately need help.i had a 3 years contract with cartrack in 2018 for my vehickle, their tracking device stopped working in March 2019 and I've been calling them requesting them to come and fix the device, they never came and I decided to move to another tracking company and requested them to calcel my contract.they are refusing and telling me that I'm liable to pay cancellation fee of R1700.noe they keep on calling me saying I'm owing them R2300 and I I dont pay dyl blacklist me.