Ask them in a low, mock-funeral voice if they worry about losing Olam Haba by being “מפר בריתו של ארהם אבינו”. (See Rabbi Brand’s essay titled: כל הכופר בטובתו של חבירו, סופו שיכפור בטובתו של הקדוש ברוך הוא).
When they threaten something about the “Three Oaths”, ask if they mean the hallowed three-fold oath to all three forefathers Hashem made to give the Jews the Land. When they grimace, act confused.
If they utter the words “כחי ועצם ידי”, tell them you now see the rule Chazal gave us for missionaries applies to them, as well (Sanhedrin 38b: כ”מ שפקרו המינים תשובתן בצידן). Why don’t they go read the pesukim inside? Translate into German (Ivri Teich) if need be!
If in the land of Israel, ask what they are doing here (especially during war, no matter how minor) looking puzzled.
For good measure, add the Brisker Rav’s other “vort”. (As Saul Alinsky taught: “Make the enemy live up to its own book of rules“.) They really hate that line!
Or sweetly remind them to hurry up and repeat benching within the zman. My personal favorite! (After all, they didn’t intend to mention the gift of Eretz Yisrael to us, and the Gemara in Berachos says this is an איבר שהנשמה תלויה בו of Birkas Hamazon.)