I work in National Parks and at one park we worked at there was a horrible problem with litter at a specific trailhead / climbing area. We put out signs like this, distributed pamphlets, even put out a radio ad pleading with people to stop littering. Then one day someone realized: it was the only climbing area in the park without a trash can or a dumpster. So we put a dumpster there. The littering problem was pretty much gone within a few days.

Should people know not to litter? YES! Do some people need their hand held in order to do the right thing? Apparently. There's assholes in every bunch, and often subtly guiding the assholes into doing the right thing is easier than trying to shame them. If they could be shamed, then they probably wouldn't be doing it in the first place.

And just to state the obvious: you're an asshole if you litter - regardless of whether there's a trash can nearby.