Move his house 2 inches to the left, so when he tries to unlock his front door, he misses the keyhole.

Let Scientology and the Jehovah's witnesses know he's interested in knowing more. Do the same with military recruiters.

Sign him up to receive junk mail from sex toy stores. Use his name and his neighbors' addresses. Maybe try to get travel brochures sent to him for countries known for sex tourism.

Why be petty and justify the neighbor... just move and never think of them again.

In the middle of the night, take a bunch of nitrogen fertilizer and lay out a "Fuck You!" message on his lawn. Water it into the ground.

The message won't appear for months until after you move away.

If his TV is close enough to the window to see what he's watching, and you can find a remote with enough range to change the channel from your own house, well, you know the rest.

Look up where the property line really is, and if the neighbor is encroaching get it surveyed and enforced the the new owner can reclaim part of the neighbor's land

Plant catnip all across your future former garden. Preferably close to the neighbour's terrain. Make sure that it's really easy for stray cats to reach their newest drug den.

Cat fights are bloody annoying to hear.

If you wanna spend money, glitter mail. or if its a house, set up a flood light pointed at their house and set it up on a random timer for only at night and blast them with "security" lighting.

Attract tons of wild life with the food you need to get rid of.

Sign up for grindr and start sending people their way.

Learn the noise ordinance laws and maximize that to the fullest for a short period

Stand on the property line constantly and try looking through their windows, or watch them, if they approach, quickly leave (don't enter their property)

Sign them up for stuff. Especially if you can find their email through LinkedIn etc.

Or, just move on since they will be out of your life regardless, don't spend your energy on them,they aren't worth it

Edit: Actually rather than grindr, send scalpers, leave good people out of these shenanigans

Plant a single piece of bamboo in a little used area near his lawn. By the time he notices it. He will never get rid of it

If Canadian, chuck a bag of milk in his eavestroughing. The heat will rot the milk and the bag will degrade in the sun till one day it fails, releasing STANK.

Sign up for a bunch of free magazine subscriptions, like Wisconsin cheese, harbor freight, etc and put his address on it. He'll be inundated with junk mail.

Throw wildflower seeds (non invasive) over onto his perfectly manicured yard.

Another one is a long con: befriend crows, get them to come to your friend's house to feed. The neighbor will likely sho them away which will aggravate them. Crows hold grudges for a REALLY long time and only shit where they don't eat, aka his yard.

Order some joke/fake lottery tickets. One of the ones where every ticket wins $30k. Drop one on his driveway before he heads to work in the AM for him to find. I've only done this to two people and they both fell hook, line, and sinker for it. One lady was calling her husband to come pick up the winning ticket to keep it safe.

Don't. We're all stuck on this little ball of rock together, and making each other miserable just makes life worse for everyone.

I don’t know what kind of neighborhood it is but sprinkling cat food or something like that everywhere would probably attract something. Your friend could even do it to his own yard. I’d be weirded out if my neighbor moved and suddenly his yard had 25 raccoons in it.

Spread dandelion seeds. You can buy them online and literally just toss them everywhere and he will never be rid of them. Or kudzu Vine that shit never goes away

When I was a kid in a second world country, you would put yeast in his latrine. That would teach him.

Unfortunately, that is probably no longer applicable.

You want to ruin someone's life? Plant some fucking greenbriar on the property. Genus Smilax. Thorny vines that grow from rhizome-like tubers in the ground. Grows a little slower than kudzu but I'd rather be around kudzu. Harder to eradicate than white supremacy. Once it's got it's little tumors in your soil, it's over forever. It'll sent shoots out just under the grass to spread out like strawberries. If you get both genders of the plant in one place, the females will grow berries that the birds eat and then they'll shit the seeds everywhere, the complicit little fuckpukes. If you don't dig up ALL the plant, it'll just grow back harder. I've seen them strangle a dogwood tree to death. They'll grow 40 feet high if they've got something to climb. There is no commercial, medical or craft use for them. The leaves have a waxy coating that protects them from herbicide. I haven't tried fire yet but it probably won't work.

Clean his room.

I heard of this funny trick where you put a bag of ice over the pilot light of his boiler. The boiler will put out gas until the ice melts and the pilot light comes back on.

And let's justr say, that's when the prank really 'pops' off

Roundup a big dick into the grass. Won't show up for a week.

If your, erm, "friend" is planning to but has not sold ~~your~~ his house, then this is an even more terrible idea that asks for trouble.