How do I keep cats from using my flower bed as their litter box?

put moth balls around.

Try keeping a litter box in an inconspicuous place outside and they will find it and use it!

mulch or pinestraw (down south), hay or even a layer of weed prevention fabric - the cats like the feeling of dirt, and think it's a cat box. Cats don't like big chunks of mulch or any sort of cover.

grab a cat sized Hav-a-Hart trap. then take it to the local Shelter! I have the same issue with 2 cats who hunt my yard birds and next week if after a notice I posted in our local Neighborhood site, they are still doing it, Hav-a-Hart! or perhaps they will become the unwilling meal of a hungry coyote )o;

Lay hardware mesh on the ground so they cannot dig.

Scatter orange peels, used coffee grounds and/or mothballs around.

1. Cats hate aluminum foil and will not walk on it if spread around in sheets. Have done this with an indoor cat years ago.
2. The county I live in has a "leash" law for cats. Call your county Dept. of Planning and Zoning and ask to speak to a zoning information person about your area. If there is a "leash" law, speak kindly to the owners and see if they will cooperate with you in some way.

Get a motion activated sprayer at your local diy store, it will spray them when they come near and kitties don't like to get wet.