Sorry for being so negative, but the "Liter Bugs" are born and raised to be messy.
It is too late to educate them.
You just got to fine the crap out of them, that's it.

Civilized fishermen will be civilized 99.999% of the time. They don't need to be awarded with anything. Their only award is a gratifying feeling of being able to fish and enjoy the clean outdoors each and every time.

What is so difficult about picking up your own junk and not making a mess of the environment?
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Know fish or no fish.