The tour group can't and won't do anything unless what they're doing is illegal, or the guide is really passionate about something.
The one thing you can try sounds painfully obvious, but hey, if it works...
Try saying something to them.
It sounds like a whinge, but some people just don't consider what they're doing or realise they may be being offensive. A quiet "Hey, this is a historic place with a tragic past, do you think perhaps you could not do [x] - it's quite offensive / rude".
Sure, some will just look at you and roll their eyes, but you're then no worse off than before.
In July when in Samarkand, Uzbekistan, a group of us spotted another with one guy in their group not wearing a shirt. Sure it was over 40 degrees C in the shade, but it's in a country and culture where you don't go shirtless, especially around the temples we were exploring! We walked over and quietly mentioned it, he had a sort of embarrassed laugh about it, but the shirt quickly went back on. We saw them later again and had a good chat, no problems, they were good people and just hadn't considered their actions.