Actually posting signs at boat launches works pretty well. I do this at Ball Pond. I make up my own sign that says, "Ball Pond is closed to fishing until April 17th". I post about six signs on trees, at the ramp itself, and on the bulletin boards.
So far I have had two guys about 20 years old launch anyway and fish but were yelled at by a water front landowner so they came in and I spoke to them also.
There were situations right after it was stocked and before the signs went up with guys fishing at the ramp. Unfortunately, we stocked the day if poured and the two DEP guys did not have raingear so they wanted to stock the ramp so they could back into the water and just toss the fish. The actual stocking point is mid lake at a beach where we can get them into deep water. Anyway, with schools of trout swimming around in two feet of water this attracted anyone with a rod in the car. But with the signs up this kind of fishing stopped.
If you have a favorite spot that gets poached a lot make up your own signs and post the opening day regulations, it will make a difference.