You need to call them or send them an email and tell them you won't your Payment Method changed to: none. Tell them you don't want to get charged every time your balance is low and want to fund your account yourself. They will understand this and advice you, once you change your payment method you cannot go back to the method you are on now (where they charge you every time your balance is low). Just tell them you understand that, and they will change your Payment Method to: "none" so you have to fund your account yourself when it reaches zero.
add any of your other credit credit with wrong cvv and incorrect expiry date/ and make that CC as default card, it will atempt each time when ur account is low, but it cant charge