NWGratefulDiver once bubbled...
First off, "getting down" on someone ... even to the point of a fistfight ... is no way to convince anyone to change their behavior.
In any case, approaching a person in a confrontational manner because you see them doing something you don't like is not productive ... that only compounds the problem. If you cannot tell someone in a constructive, non-confrontational manner that they're doing something dangerous, then you're better off not speaking at all.
It just doesn't always fit the situation.
Sometimes, the drunk guy just won't hand over his car keys.
I'm more concerned with preventing an imminent accident that changing anyone's behavior.
One time, I was walking to the transom to dive The Playground, off West Palm (135fsw) when I noticed the nitrox label on the tank in front of me read 38%. I asked the guy what his MOD was, and he had no idea what I was talking about. He was obviously diving nitrox without a cert.
What, exactly, would you do in a situation like that?