I have been known to announce to the offending parties: "WALK!"
I hauled a kid over to his neglectful grandmother a few months ago when he was climbing on a display of tomatoes and pretending to shoot my boys with a banana. She just looked at me and skulked off with the little monster.
My kids were appalled at both his and her behaviour.
Me, too, but I want to stir up (or at least embarass) the parents, if they're within earshot :-).