Paintball gun might work. Or maybe mail the neighbors fence brochures.
Paintball is good because 1) it'll hurt like a sonuvagun, yet won't seriously injure the animal and 2)will provide visual cues to the owner that the animal was where he shouldn't have been.
Animal control should also have some live traps available. They're kinda scary looking, so maybe just having it in the yard will freak the neighbors out about what your intentions really are toward their dog. Muahahaaa.