How the hell do I disable automatic billing?!?

Simply put, you can't. Linode invoices you automatically on the first of every month, at approximately midnight US Eastern time (UTC-0400 at the time of this writing). For users on metered billing, there can be an additional invoice or two mid-month when the amount of outstanding services reaches the set limit. Your card on file is automatically charged whenever your account balance drops below zero (usually as a result of an invoice you just received). I've personally never seen a bank that subtracts the amount of the charge from your savings account as a result of a transaction against your checking account that has insufficient funds. I'd suggest contacting your bank and beating them over the head with a cluebat, because they're supposed to decline the charge in that case.

Edit: it's worth clarifying that an invoice is not a charge attempt in and of itself. When Linode successfully charges your card, you'll get a payment receipt email, which may be anywhere from a few minutes to a few hours after the invoice, as there's often a delay in between the invoicing and the charge attempt on the first of the month billing (unless you're on metered billing).

Open a ticket, while linode staff do monitor these forums, tickets are answered pretty much immediately.

@dwfreed:

I've personally never seen a bank that subtracts the amount of the charge from your savings account as a result of a transaction against your checking account that has insufficient funds.
Some banks offer it as a "service", i.e. just another excuse to charge more fees. OP should ask his bank to disable it. The only time it's even remotely useful is if you have overdraft enabled on your checking account and the overdraft fee is greater than the withdrawal fee of your savings account. Which, of course, is a really bad setup in its own right.

AFAIK the only way to prevent Linode from charging your card at the beginning of each month (or the end of the month, depending on your time zone) is to "top up" your account balance ahead of time. So if it's more convenient for you to pay on the 15th day of each month, figure out how much the next month's invoice is going to be, and prepay it on the 15th.