neighbors’ dad insists on mowing their lawn at least twice a week, especially when I’m trying to enjoy my hammock.

Noise-cancelling headphones.

Maybe try buying him a hammock? All the other ideas lead to either a lawsuit, someone’s death or simple violence. Because littering his lawn with bike chains would be dangerous for everyone.

riding mower

That’s your problem, they’re designed for people who want to waste time on their lawn and drink a beer. People who want to get it done with have zero turn mowers.

Sodium silicate in the oil. Bleach in the gas. An iron spike driven into the ground just below the height of the grass.

But really the best way to deal with it is to get over it. I know sometimes it’s hard to get over an irritating sound, but it helps your mental health if you don’t get angry about something minor and entirely outside your control

Have you tried talking to him?

Do not cause permanent damage. Absolutely don't. Not worth it.

Try to sell him on something quieter. If he absolutely wants a mower he can ride on, try to sell him on a better and quieter electric one.

If he absolutely won't change, see if a noise complaint is possible (get a logging decibel meter).

And last, just unplug stuff. It can be plugged back in and it's not destructive, but IS very annoying

If it uses a combustion engine, you could try putting something hydrocarbon-soluble in the fuel, like polystyrene. Some of it will get past the fuel filter and foul up the carburetor (and probably the spark plug) without causing permanent damage to the engine. Removing the blockage is trivial, all you need to do is soak and rinse the parts in fresh gasoline, but the diagnosis and disassembly requires some effort and technical knowledge.

Also, I think we all should acknowledge this is Dullsters. ITT it's violence all up and down. Come on, team.

For around $1 million USD, you would launch it into orbit.

This would not cause any additional wear and tear so it would look pretty normal (through a telescope).

Scatter stones all over the yard being mowed so it fucks up their blade.

Project Farm to the rescue: https://www.youtube.com/watch?v=IsjKLqUnwyU

I think lapping compound destroyed it the fastest, IIRC.

Remove his spark plugs.

Loosen the screw that’s holding the blade in place.

If you can get to the mower just put sugar in the gas tank

Scatter some flower seeds in the lawn.