Yes, it’s a major problem – I would put fouling by cats alongside attacks by slugs and pigeons for aggravation! The only thing that works for me is to put down physical barriers of one kind or another to stop them. I cover most of my beds – especially those with any friable mulch or soil on – with the sticks I will use later for peas, and later in the year I use the cut stems of the autumn raspberries during the winter – they’re more bristly than the summer ones, and the cuttings from my cornus shrubs at this time of year or from the hedge when I cut that later in the summer. Otherwise, short sticks stuck in the ground to form a “hedgehog” do the trick on small areas but you have to set them too close for a cat to comfortably sit down (no more than 2-3 inches apart) or they’ll get in there! Freshly disturbed soil or fresh laid mulch or compost seems to be the most attractive to them. Once it’s settled and been planted they tend not to be so keen but you can’t guarantee it ever! I agree with other contributors that wet soil is less favoured but you can’t always water everything all the time even if you wanted to. When you net crops against butterflies or other pests or against bird damage, you obviously cover the ground anyway so you do get some respite. But I’ve had them crap on my dug potatoes as they lay drying off, and on a low thyme bush in mid winter when there was snow elsewhere! If only cat owners would train them to use a litter tray indoors (as my son’s cat does). It really does make life difficult and adds to the labour of getting seeds/plants into the ground and harvesting them in an edible condition, plus it’s such fun to find when you’re happily weeding on your hands and knees!! I have lightish sandy soil so I guess I’m more vulnerable than someone with clay?
I have found electronic devices expensive and not very effective – as plants grow they block the signal anyway and it’s only really when you’ve got little else to get in the way that you can cover the area with a workable signal….