We found the ultrasonic cat scarers worked very well on our tiny front garden bit (which was covered in pointy gravel) and on the back garden when we moved into our last but one house. We moved with them to the new place – but cat poo has never been an issue at this place.
One theory (from our cat owning neighbour) was that a lot of the crapping is territorial – a bit like a Singletrack thread, actually. If cat A craps somewhere, cats B, C, D, E and F through to Z feel obliged to also crap in the same place. Stop it all at once, and it stops being a problem.
We found that once all the cat poo has been cleared away (yay…) and the cats successfully scared off, they tend to leave well alone.
By the way – those big Nerf darts make an eerie whistling noise.