It depends on the terms of any agreements you have signed with the company, including either the old policy or a current one. If you signed anything--or otherwise agreed to--giving them the power to take money from your checking account for any past due debts, this is likely legal. On the other hand, if you never agreed to this, it is most likely illegal. Therefore, you need to check your communications and agreements with the insurer, to see what you have agreed to.