Unless you have actually seen the cats,it might not be cats at all..
I often find BIG turds on my lawn,I even picked one up today,it's from a very large raccoon that comes around here every night,as well I have skunks and opossums.
Cats seldom do there business without covering it up,usually no smell is left behind,that's why they keep covering up and sniffing and if they go in your garden,it's good fertilizer
I don't know of anything that will keep any animal out of your garden,in my view I just have to share my piece of nature with whatever lives out there and I would certainly never harm anything.