They can’t force you to work it off, they have no authority to enforce such a thing. If you walk out without paying, they can call the police and file a report. You still owe them, so they can sue you over it. Realistically, when faced with the court fees and logistics, any sensible restaurant would drop a one-off offence, even if you stood your ground and refused to pay, because the amount of money is simply not worth it.