Honestly? These people are a good inspiration to lift and get in shape.
Publicly disagree with them as firmly as possible. Hell, call them a gammon or a cracker to their face, and watch them freak out and go apeshit, and if you're physically stronger than then, have them have to deal with the fact they probably can't kick your ass and get a standing ovation online.
I'm already somewhat ahead of the curve there, at least for an old guy. I could be more aggressive but usually I just listen then start to slowly shake my head until they stop.