A guy at the star party in Flagstaff almost ruined my best shot

I was out at the Buffalo Park star party last fall, trying to get a clean 30-second exposure of the Andromeda galaxy with my new ZWO camera. This older guy, who seemed to know his stuff, walked right up while my shutter was open and clicked on a super bright red headlamp to point at my tripod, saying 'You know your polar alignment is off, right?' The whole frame got flooded with light and was totally wrecked. He meant well, I guess, but he didn't get that even a quick blast of white light ruins dark adaptation, and red light can still mess with a long exposure if it's too bright. I had to start the whole 20-minute sequence over. It taught me to always, always have a polite but firm line ready to stop 'helpful' advice during a live shot. Has anyone else had to deal with this at a public viewing event? How do you handle it without being a jerk?