The Dog Next Door

My neighbors have a beautiful black lab which, through the wonders of the modern day doggie door, has free and unfettered access to their backyard. This dog also has an abnormally large pair of lungs. How do I know this? Because it barks nonstop. What is it barking at? The wind. Birds. The grass. The silence. Random voices I'm sure it's hearing in its head. It's driving me nuts. Any suggestions on dealing with this? I've contemplated shooting it with my pellet gun. I've got a sniper position picked out in my upstairs bedroom window. Hot water tossed over the back fence--I'm sure that would only serve to whip it up into an inconsolable frenzy. Tainted meat? Hmmmm...