What is really needed is a concerted, determined and on-going education program. Start with the children and then work on the adults.
I've had many experiences where locals were throwing their garbage on the road or the beach. My protests were most often met with derision. In one instance, the response was "what's the big deal? Look at all the trash that's already there?" In another instance, the reaction was less nice than that. It was more along the lines of how a tourist shouldn't be telling a local "Don't throw that there.", as it is "none of my business".