Altech Netstar is de*****ing me

I took out a contract with Altech Netstar for a tracking device in June 2012. This unit was transferred to two other cars and finally my current one. The contract expired end of June 2015 but Altech Netstar continues to charge me a full rental amount which they have since increased. All this despite the fact that I reminded them of the expiry. I was even given a reference number CAS504280R1V1 for the query. I fail to understand why they cannot keep track of their clients's contracts. If I had not queried, they would have continued charging me rental for a unit that I now own since 24 June 2015. If you are failing to sort out such a minor issue, what makes you think you will be able to handle bigger issues?