I let most everything slide, but if people are talking non-stop I ask them to go to the bar to talk and I do that whether I am recording or not.
My wife is excellent at that — I need to learn how to do it myself in a way that is calm and not snide. (My kneejerk reaction is to say something like "Can you speak up? I can almost hear the music over your conversation," which isn't helpful.)
My sarcasm usually screws me in the end, so I just try and stick with dirty looks. I will, however, have a conversation with my immediate neighbors before the show starts, saying things like... "I'm glad I'm next to you, you look like a normal person who's here to listen to the music and not scream and clap in my ear and talk through the show." I try to get a conversation started about how some people ruin the show for others around them by talking, whistling, screaming, over clapping, etc., instead of actually listening to the music that we just paid way too much money to hear live. Depending on how things are working out, and if they seem cool enough, I'll tip my hand and tell them I'm recording it (assuming it's a stealth show), and if the crowd is quiet around me, then I'll post the show so everyone can listen to it again as much as they want...but wouldn't bother with it if I feel there's excessive crowd noise. This essentially puts it on them to be cool or assholes. I've also said to people when I'm open taping, "I hope you're not saying anything that you don't want the entire world to hear, because you're all over this recording." That often makes them think for a second, and they'll often walk away.
Another idea, which plays off of what the OP said, is to use a second rig of just a small deck and a pair of omni's, a little lower and designed to pick up the crowd (diversions). Introduce yourself and get names as well as covertly snapping a photo or two. Post that version of the show, thanking the people by name for ruining the recording and supplying the photos to essentially out the noisy people. Those recordings would make the rounds and it just might aid in curtailing crowd noise around our mics in the future.