Anger is just a stage. Carry some small trash bags and pick up a little. Hand some bags out. If you see someone litter, go pick it up yourself and then look them right in their face and smile, say nothing and move on.

I've found that people are reluctant to litter on a clean beach yet unthinkingly do it if the beach is already visibly trashy. So the strategy is to get a beach clean enough where it is obviously not OK to trash it. People will get it.