Service sucks

I am a new client to Cartrack, and when I first signed up, I specifically asked them not to debit my account right away as there was no funds available, and that it should be debited last day of the month, 2 days after installing the devices in both vehicles, my account was debited. A week later they tried to debit again. And when received my salary last if the month I received +-R400 less in my account due to all the unpaid reversals. And now they keep on calling me saying I own them money. If I could I won't cancel immediately pity it's a 2 year contract.