Conundrum

So Mark and I live on the South Fork of the Snoqualmie River, high up above the river. He owns the property from the house all the way down to the river but you can't build down below because it's a flood plain.

So here is the conundrum. People come from across the river and set up little fishing parties on his property. They seem completely unaware that it's like if we came to their house and set up a Hibachi in their backyard. It's pretty far away so it isn't too intrusive in our everyday lives, it just seems a little arrogant. There is no road or trail to get to the little sandy beach and you can clearly see the house. Tonight there were about six people down there with chairs yucking it up and fishing. It seems unfriendly to put up a "No Trespassing" sign but at the same time it feels a bit weird to have them there. Maybe "Smile, you are in our backyard!" What do you think?