Walk/drive up to them and tell them they need to clean up after themselves, or better still, chuck their litter back in their driveway or porch or wherever.
It was only a couple days ago that I saw some kids (little ones) throw their coke cans out of their parked oriental breadbox, and their father just sat right next to them smiling away like a total douche. I know I shouldn't have, but I couldn't help myself from walking up to the guy and asking him to clean up his sh*t off the street. The good thing is that he actually did it without making a fuss. I'm sick of these scum bags who ruin public property like their trust fund pays for it.
End rant