Unauthorized Debit

I contacted Netstar in April and informed them I can no longer afford the premium for my tracker unit so I would like to cancel it. They refused me to cancel instead they said I can downgrade, In June I downgraded and to date, they are still debiting my account with the old premium. At first, they told me I need to pay for the July premium with the old amount. It has been the up and down of calls with no solution to the matter. I have spoken to more than 10 consultants that cannot help. It is 4 months now, and they keep on going to debit my account, and there's no money they are causing me more banking charges. Since this cannot be solved I want to cancel this and I will look for something that I would afford.