Cats using my garden and POTS as a toilet - I've had enough! Any tips?

Besides getting a cat myself - not really in position to do so at the mo.

Look, I know lots of cat-less people here have had this problem,and I had it in the garden last year, but I'm especially yucked out today. I have some pots and troughs on the patio that I use for growing salads and veg (nothing planted yet, but compost in) and I went out this morning and they were FULL of cat-shit. It's repulsive. Is there anything I can do beyond covering every bare piece of earth and unfilled pot with chicken wire or similar? I appreciate cats have to go, and location is nature an' all,  but I'm SO sick of having to clear up/bury their crap. I've seen various cats in my garden (never caught in the act, usually just wandering through) so it could be any one of a number of poss culprits. Arghhhhh!

Oh, and PS. Is it considered cruel by owners to get litter trays for their pets? because 'the neighbours' gardens' isn't really on as a more 'natural' alternative in my opinion. My local cats also do not seem very good at burying their doings so I do end up having to remove it. It's gross.

OK. I'm all ranted out now...any tips appreciated. Ta Lx