Make sure your wifi is wired to the wall, then your neighbor's crime counts as wire fraud. As a private, natural person, you might also look into some loopholes in the Uniform Commercial Code.
Even if you manage to stop the WiFi hack he can still get in to your network through the ethernet ports. He could train an army of ants to pull a wire through a small hole in your wall and attach it to your router.
Just in case, fill all your unused ports with yoghurt. This will drown the ants
Easy, buy a nice second hand microwave, remove any shielding from the rear part. Ask a skilled friend to make it microwave stronger. Put it as close as possible to neighb’s wifi. Switch it on, leave it on, hacking is now fixed.
Ah, the age-old wifi-hacking-through-photosynthesis conundrum! But let's not jump to conclusions—perhaps your neighbor just has an insatiable thirst for bandwidth... and possibly your beer.