Cartrack bad service and deducting unauthorized amount

The company has been abusing me for too many months not allowing me to cancel my car track after I sold my car, They kept deducting and we had an agreement that they will deduct an amount to settle the account , after they deducted that amount they kept calling and telling me to pay some extra amounts for the last time when asking where is the cancellation letter they told me to wait for 20 days they said they will deduct R180 as the last amount then everything will be sorted but they deducted R500 I called and asked they said they are escallating my issue they will send my enquiry to their billing or finance department to reverse my difference which was never paid back. On the 2nd of February they sent me a cancellation of contract letter, but they still call me and tell me to pay some money whereas they are the ones owing me and the caller that called me denied that they sent me the cancellation letter and she says I must send her back the letter of cancellation, Car track has bul**** me for too long even today I was receiving their calls. I thought after the cancellation letter and the amount they made me to pay I will be free from their abuse. I receive calls daily with the same type of abuse.