You might want to consider writing a handwritten note, laminating it, and posting it right by where they get to your pond. Emotional appeals might deter some people that will ignore, or get a thrill out of, walking by a "NO TRESPASSING" sign.
"I've worked very hard to develop this pond for my family's enjoyment. When you come onto our property, you damage what we love and destroy my hard work. Please do not hurt us by trespassing on our land."
I am actually considering this option too. I know within a 5-6 house range where the teens are coming from. I thought about taking pictures of the damage to my fence line and writing a short letter that communicates the fact that we're investing our time & money into repairing damage that has been done by trespassers and just want the violations to stop. Then putting these fliers in the mailboxes of the houses on this street with hopes that the adults in the home are responsible and can say something to the teens.
But that said, I also figure that can open up another can of worms by essentially "advertising" the property and also getting the teens in trouble = opens up the odds of them vandalizing the property.
