Say something to them about it. Don't be a dick about it, they probably just don't know any better. Or, they do know better and are too damn dumb or douchey not to do it.
Park douches are why I rarely ever dip into the park and I stay well the fuck away on the weekends.
"People say that marijuana smoking is going to get in the way of my career. I say to them that on the contrary, my fighting career is getting in the way of my marijuana smoking." -Nick Diaz