tracker do not cancel contract when asked to

in April 2015 I sold my car and contacted Tracker to transfer the system to my new vehicle. I was informed I cannot do it and I will have to buy a new device. I said I then would rather cancel my contract. I was informed it will be cancelled. on 2 may I realized the premium was deducted again and I contacted again to find out what the problem is. I was informed the system could be transferred but I declined the offer and again said the contract must be cancelled. I was told to pay an additional R400 cancellation fee. I told the person to deduct the money and get it over as I felt and still feel they rip me off. I do not want to pay for something I do not use. I received notification that my car is stationary and it seem to me the contract is still not canselled. I phoned and was informed another installment will be deducted the end of the month. this seem to me to be theft. I did cansel the contract more than 30 days ago.