BMW FALSE DEBITS

I bought a BMW through the bank not BMW. BMW Financial Services is debiting my account every month with different amount, I NEVER EVEN AUTHORIZE THIS - which i feel it is *****, they are *****ing from me. I took it up with them, they did all the checks and was told it is a mistake, they will refund me. THEY ARE QUITE AND ANOTHER DEBIT WENT THROUGH. What should i do?