Harassment from attorney - Cartrack

I have sold my car a year ago and I requested Cartrack a month after they have installed the device to remove it. They have not came to look at the device as I requested. A year further they send out a technician. But the car had a malfunction and he told me this was the battery. I changed it to a brand new one and still the problem occurs. So I sold my car with the device now they still charges me for the device. I have send them emails. Called the explain to them but they still don't stop. I m a pensioner now and explain to them that the car is now sold. Please i need them to stop all action with attorneys.