You could leave, or plug in earplugs, or ask them to stop, all of which might be better than suffering to prove how much you don't suffer!

Yep. This is one area where I differ in application from Byron Katie; I tend to focus heavily on self-applied judgments -- i.e. "I should(n't) X" -- rather than other-applied ones. So in AnnaSalomon's story it seemed to me the real problem was the thought "I shouldn't be petty", since there didn't seem to be any moral judgment being levied against the muncher, vs. against herself.

That being said, Byron Katie is correct that it's a lot easier to work on other-applied judgments and that it's better to learn the method using those first.

(I also sometimes find, oddly enough, that when I get to "who would I be without this thought?" on a self-applied judgment, my mind will sometimes object that if I didn't have this thought, then I'd have to stop being mad at other people for doing the same thing that I'm upset with myself about! I then have to reflect on whether on balance it actually benefits me to be upset at those other people, considering that it rarely motivates them and that the self-judgment is impairing me.)