A motivated attacker need only don a green safety vest and hard hat, then roll up with a white pickup truck, place some orange safety cones and take down the sign with a chainsaw.
Note that in the article, the author says how the scammers do everything to avoid having to show up in person. That's because they are in a different country and try to commit the scam without setting foot in the US.
The point is that nearly all of the people doing this don't even live in the country where the land is being sold from. A simple sign would probably be quite effective