Evil neighbor girl

So our backyard is very secluded.....but behind our yard there is a girl who is about 7 and she and her friends took to yelling my son's name through the trees early in the summer. Nothing came of it. He is nearly 12 and not wanting to be bothered by the likes of a seven year old girl. So tonight he's in the pool and I'm out on the deck and she starts creeping through the trees and bushes to be seen by him and says "Hi! (my son's name)" and he ignores her and then she keeps giggling and standing there and he fills a water gun and squirts her. She is in a bathing suit but I still said "Don't squirt her" and the girl says "Its ok! I'm in my bathing suit! Its fun"..ug. SO he still squirts her and she stays. She keeps saying "(son) is so silly isn't he? Teenagers!!!" and tells him how silly he is. Then she manages to keep walking around the above ground pool holding on to the side and jumping so she can see ---which makes me want to scream but in stead I nicely say "Please don't do that honey--the sides aren't supposed to be pushed against"...and she says ok but still does it. Finally I say "Let's cover the pool (son), it's getting late" and she still lingers...and my husband comes out to help us cover the pool. He nicely pats her on the head as he walks by and she starts swatting his arm with her two hands!! Like she was reacting to his little pat on the head--as if he'd hurt her..and she makes this face and says STOP IT! and he looks at her like she has two horns. Looks at me..and my eyes widen and I say "(my son) has to go inside now..so you probably should go home"...Darn it we need to fence in the yard!