I'm rapidly developing a reputation as a Dune Nazi around the Gulfview Heights walkover in SRB because I have no hesitancy to put my beer down, climb outta my beach chair, and tell people the dunes are off-limits -- especially when it's ADULTS sliding down the dunes with kids.
I must say, though, I have never gotten a resistant reaction from anybody. In many cases, you see a light bulb go off in their head and they sheepishly apologize, look away, and say something along the lines of, "Oh ... sorry. I didn't know." I'm nice when I approach them -- but fully prepared to get stern, if necessary. So far, no need to be stern.
I remember being a kid vacationing on Cape Cod and climbing up the dunes in West Dennis (ironic name, huh?) then rolling down as if they were a snowy hillside. It WAS irresistably fun but that was 40 years ago and we've learned a lot over the years about the fragile nature of the dunes.
I would encourage even the timid types who see dune marauders to be assertive. Take the approach of educating those who are simply having innocent fun -- but doing a lot of damage at the same time. All in all, I think people no more want to hurt a dune anymore than they'd want to hurt a puppy. Sometimes, they just don't think.