Look up ultrasonic animal repeller. I've never used one, but it may work for you. It makes a high pitch noise that should stop cats and other animals from wandering into your yard.

Partially jokey answer: An automatic sprinkler and a motion detector

https://www.youtube.com/watch?v=ElcviGYMb3U

Serious answer: I don't think there's much... I used to have a problem with cats pissing on my bicycle (which used to be parked outside) and the only thing that would keep them away was a bike alarm - at the expense of mine and my neighbors' sleep. Covering the bike made no difference, they just pissed on the cover instead. The fix ended up being just parking my bike inside.

If you let the garden get a little bit overgrown, do they still come and drop their shit there?

My family drops mothballs which isn't an environmentally friendly solution, but it's more humane than what my stepdad wanted to do.

I saw a guy on YouTube make an automated water turret that shoots the water hose (not a water gun, but the actual hose) at anything that the camera recognized as a cat on his lawn. It was awesome. I tried finding the video, but can't remember his name.

Side note on the chili idea, it doesn't really work on many shorthaired cats. Neither does cayenne pepper powder. [E] That was a problem I had at a previous house. I had a neighborhood cat that kept meowing hella loud in the middle of the night right outside my window. The cayenne and chili actually attracted him and more cats, and I ended up with three visiting cats, instead of the one original. Just be aware this could be your fate, too, if you use foodstuffs.

You need to make your lawn less appealing than your neighbors. They might not care as much or even notice the cats shitting in their yard. A plastic owl works with birds... maybe a lawn ornament of a predator would do

Shit on your own lawn to establish dominance.

Yeah, I saw pictures of how Roomba smeared dog shit all over the floor once, so I can imagine how a robot lawn mower could make it worse

Go xeric and nuke the lawn or motion activated sprinklers.

Cats can be territorial. Shut on your own lawn. Claim it for yourself.

Get a dog.

Sorry buddy it’s their lawn now, smells like them.

Don’t like it? You gotta shit on your lawn to show them who it really belongs to

You can have a dog, but then you'll have a problem with dog shit

Bushmasters are good for this.