A. Put up a sign
B. Don't put up a sign but ask your customers how they're planning to pay before they order
C. Wait until the subs are made and then stare at the offered debit card. Inform the customer that the machine is broken. For bonus points, suggest that the customer go over to a nearby convenience store to use their ATM and pay finance charges for doing so.
If you picked C, congratulations, you're a moron. Hope you forever work at Subway.