Gadolhadorah
I know it was meant as a lighthearted attempt at humor, but I still feel I must protest.
As a proud “Litvak”, I take umbrage at the insinuation that I would attack someone who comes into my Shul waving a yellow flag and singing “Yechi…”
What I (and, I think, just about everyone else who Davens at the same Shul) would do is either ignore the guy completely, or give him a friendly greeting and offer him a drink or something (depending on what else I was doing at the time).
I certainly would not take any action against the unfortunate fellow.