there are two solutions.
1. fence
2 Start piling the cat poo on your neighbors car or front step.

I've been using 1/2 drip line as hoops across my boxes and draping plastic bird netting over it. Of course I trip on excess netting all the time but it sure keeps my cat and the neighbor cats out, not to mention stopping the birds from eating my seedlings.

shotgun

no seriously

rifle

no seriously

anyway, I have 2 cats that I know are going to do this, let alone the umpteen friggin neighbor cats... I THink a 1 foot chicken wire fence that I can just bend up when not tending and bend down to get access will keep them out....

I just constructed two 4X4 SFGs and the first day that neighbors cats were all over them. I decided to make my grid out of twine tied around nails at 1' intervals around the bed frame. The twine grid seems to have made my beds unattractive to the cats now! Guess they don't want to step over the string.

Ball up foil or you can lay out flat in empty spaces. Works for eveytime... I have three cats

Buy some chicken wire -- around $5 for a roll. Cut into 1 foot by 1 foot sections and place over the squares as you plant them. I use 1x4's as my grid, so the chicken wire fits perfectly onto the grid. Sections cut larger can be made into arch shapes to allow plants room to grow. By the time the plants start to fill up the squares, the cats seem to have given up on using it as a litterbox. This is the only thing I could find that worked.

See pages 65-70 in the new book. Lots of pictures. Cheap, lightweight, and easy. The book shows the "cage" sitting right on top of the 4X4. As another option, you can build the cage so that it fits over your garden and sits on the ground...no cats this year!.....

You can also take plastic forks and stick them handle side down and tine side up in the beds. Use different colors and be creative when you answer questioning looks! Be careful you don't stab yourself!

I had the same problem, I solved it simply by cutting clear plastic 2 liter bottles in half (poking holes in the bottom half) and placed one each over planted seeds. once the seedlings become large enough you remove the bottles, and the cats are disinterested in a garden full of plants. the bottles also help keep humidity high while the seeds germinate.

unplanted squares are always prime spots for cat-poo. Laying down chicken wire would definitely solve the problem, if the cat cant dig, it wont want to use it as its litter box.

And another thing I forgot to mention: In the past I have built a separate, just for cats garden. Just till up a patch of soil somewhere far enough away from your garden so the cat doesn't see it as one big play area, but close enough so that if he's in the area and has to choose a place to go, he'll choose the appropriate spot.
just keeping a couple patches of loose soil in various spots in the yard keeps the cat disinterested in the edibles. Hey, you could even plant some catnip near the designated cat area, just for extra incentive. ( and might I suggest starting seed indoors and transplanting when you have a pretty big, hardy plant; otherwise the cats will roll on them till they're squished down dead)