How to Handle a Neighbor That Takes Your Rocks

My next door neighbor was making a fountain feature in his front yard to cover the spot where he couldn't go grass. He took one of my big decorative rocks and put it in his fountain, which honestly whatever, it's just a rock. I told him he could take all he needed. You see signs sometimes, especially in this area, like free firewood like firewood free or like on a couch, right? You'll see like free when that really means trash. Yeah. That number will never stop amazing me.