I'm a real estate broker down here in Mass. but I'm sure this law is true in NH as well, it's a fairly basic law. The property owners own the land. They can do whatever they want to do with that land, as long as it doesn't break a local, state, or federal law (some towns call them ordinances). Unless there is an ordinance/law that prohibits the intensity of a light, which I doubt, you're out of luck. Which is a shame because they are destroying the enjoyment for you, I, and everyother patron of the lake.
__________________
"I hate women because they can always find the remote, car keys, the dog, etc."