If you see what car they drive, pick up the stuff and leave it next to their driver side door w/a (ahem) polite note asking them to pick up their crap.
If you see somebody throwing stuff down, (and with a decent number of people) yelling "PICK UP YOUR TRASH" might give them the hint... just be careful, you never know what kind of person you're dealing with.
Nothing like fishing a clean beach
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