I guess their plan is to simply let the plane and all the passengers sit there on the tarmac until the passenger leaves. Peer pressure.
Watch the fights between passengers then as one gets pissed off and lets the offending passenger know he is in a hurry and the offending passenger has to leave now or else.
It's expensive to hold up flights like that but what choice does UA have now?
United had an excellent choice. A couple had already offered to deplane, but they wanted a few hundred more than United was offering. First, negotiate with the willing couple. Second, pay the agreed upon compensation. No scene, no bad publicity, just everyone satisfied.
That’s how a good business is run.