It's unfortunate I,A that you don't value yourself enough to express your concern to your neighbor/s. What gives? Whether they can't or won't (both strong words with heavy implications) that shouldn't deter you from making them aware that their fury paw monsters are shitting and pissing on your produce.

Second to that, I remember as a kid, we had a small carrot patch and my dad would make me collect twigs and pine cones and all sorts of stuff to blend in with soil. He always said it helped deter the neighborhood cats from using it because they didn't like that the soil wasn't soft. Not sure if it worked, but I sure remember collecting twigs.
Talk to the owners politely and ask them to solve the problem their cats are causing. Be stubborn about the fact that you have a right to a garden that isn't full of biohazards. Repeat how gross the poop is and how you want the pooping to stop.
Dear urban gardener: You are missing the point. As you sip on your $9 latte and brag about growing your own food, please be reminded that growing your own food takes work. That includes guarding against local pests. Its called a fence. And while you are cultivating a sense of global awareness, think about how toxic it is that your dog shits in your neighbor's driveway and their cat shits in your garden and you all just continue to have stupid cookouts without ever saying a truthful word to each other. That's what I call a toxic environment.
Yeah, build a fence. Cuz fences keep cats out. And the bbques? They better be truthfests from here on out or you're just missing the point of bbques and life according to your moral superiors.
If you talk like you type.... that may be the first problem.
chicken wire laid flat on your garden protects it from cats when the plant starts are small.
Don't listen to Douglas Banter. He's a fucking moron and will be reported for animal cruelty.