canceled service with Tracker, yet they are still debiting my account.

For almost two years, the tracker system has not operated in my car. After trying in vain to resolve the issue over the phone, I chose not to pay for something that was broken, and they sent my account over to their legal team. I contacted Tracker to cancel my account on November 23rd after paying their lawyers the outstanding balance. December an amount of R300 was debited from my account. My contract with them was canceled in November, right? They now want R300, knowing full well that it will add up if I don't pay it and that I will get into legal trouble once more.