> So the computer picks 8 people, and you call out the first four on the list. One of them refuses, just like what happened here. Now what?

You inform the refusenik that he is trespassing; if he continues to refuse to leave, you inform him that police will be waiting for him at his destination, then call out the fifth name on the list. When you arrive at the destination, he goes to jail, while the four bumped passengers get vouchers and fly later.