From what I know netflix doesn’t deduct automatically from debit card but when credit card is added then it automatically renew when the payment is due to learn more click here for payment and to learn about cancelling automatic payment click here
Keep in mind that anyone sign in will be able to change your plan and there is no any chances/way to see who actually change it. Asking respective member is only the choice. To avoid such things inform other members that if found changing the plans password will be reset and no one will be allowed to watch it. (May not works )