Here's what I used when I had a similar problem with neighbor's cats years ago, doing the same thing in our rose beds (and I love cats):
Get a bunch of CHEAP mouse traps (NOT rat traps). Load them (without bait), and simply cover them with the normal tree leaves and/or bush leaves which are usually littered about the area. Even if they don't get caught by one of the traps, the snapping sound will scare the hell outta' them when triggered (I didn't say crap 'cause that's kinda' the point of the traps - to NOT have their crap in your yard).
Years ago when I had to use this technique, it worked WONDERS, and I even found one or two mouse traps out in my front yard as they escaped the mouse traps clipped onto various limbs (to be honest, I tried to lighten the snap pressure to help keep the cat from getting injured and so they could escape the traps easily) as they shook them off (I actually saw one of them flipping around all over the yard trying to escape the trap until it released - I NEVER saw that cat again).
I kept count of the traps in my planter boxes and never once lost a trap (assumingly stuck to a poor, suffering cat). I suggest you do the same thing if you choose to use this technique. This way you can say NO CATS weren't able to quickly escape the traps (really - you might want to weaken the springs depending on the cats in your area - if they're a bunch of hardy, fighting TOMS, document it as such and leave the springs alone (as long they wouldn't cause damage to your own finger). After all, what you're after is enough snap to scare the crap outta' them (on their owner's property) so they don't want to trespass on your property anymore, and NOT to hurt them. Remember, I love cats, and I'd prefer you not hurt one of my favorite predators (yet I recognize they CAN be a problem/nuisance).
The results of MY using this technique were quite dramatic. The cats IMMEDIATELY stopped using our rose beds as a cat box. I also made sure to keep the rose bed covered in old rose leaves, so they would have NO idea if a trap lurked underneath!
I hope you're as successful as I was in creating your "cat-friendly" cat-box deterrent!
Good luck!