Intercept and deflect. Physically intercept since actions tend to get in faster ...

"Um, can you not do that? We usually do X, let me do that" is meek enough it doesn't tend to offend people.

And do other people seriously discipline? I've never seen it happen, but then we've never left DD#2 alone with other people and DD#1 is the alpha personality type and prone to telling other people what to do, including adults, and people tend to just watch her at it and laugh. I have frequently seen other parents rescue other people's children at playgroup though, when they fall over, get upset, get caught in something, have a dispute over the car (everyone wants the car) at kindergym etc but not discipline. They usually try and enforce sharing "no no, X hasn't had a turn, you can wait a little bit" to whichever child is trying to hog the equipment whether it is their kid or not. Is that discipline? Its really nice I like the people in this town.