Pathetic service

A tracking device was fitted in my husband's vehicle which served no purpose as he already had a tracker. however, after having cancelled it telephonically, he was never told to complete an actual cancellation form. he is to date being harassed by the company demanding payment, be it via calls to his mobile and sms'. Eventually a cancellation form was completed and emailed and the amount which was given to cancel the service was incorrect, but the company refuses to accept responsibility for their error. Now my husband is being harassed after wanting to do the right thing to make arrangements to pay off the amount owed, now the company either ignores our emails, gives us different balances owed or they're forwarding me the cancellation form which has been sent, along with proof of payment, to the company on numerous occasions. All we need to know is how much are we owing... but even this is too difficult... so do they work on the same system if they are able to produce different amounts owed by my husband? This service is beyond pathetic!