All these replies but the FIRST one I've seen that has a really friendly and low-impact, low-cost and benign suggestion is the one directly above my post.
It's the chili powder.
Spread a slew of it all over the area and any/every cat that enters that spot will get chili powder on his feet. No way out of it.
And the same cat will bathe itself shortly after getting any of it on their feet. And during that process, the cat will quickly put 2 & 2 together and never set foot on that spot again, for fear of chili powder on their feet.
Nobody has to get shot, nobody ends up with an MWAG call, no cats have to be physically harmed, and it's extremely low cost and doesn't require monitoring and elaborate planning.