Fire them? How? The manager was short staffed as it is. If he fires them, then hamburgers don't get made.
Then compromise I guess huh? If you have a backbone as a leader and prove you won't stand for subordinates not following instruction then it's surprising how quickly they fall into line.
No one dragged them in there to get a job so they must be there because they want money. Threaten to take it away an usually they respond well. I'd rather do any job by myself than to have worthless workers earning money they aren't REALY deserving.
The customers will wait or they won't. The problem will correct itself fast.