Your flower bed is a beacon to neighborhood cats. There's really nothing for them not to like. Soft, loose earth makes for the perfect potty for them, resulting in delivery of deadly urinary salts to your precious plants. Check on the ordinances for your area to see if leash laws are in place. If so, ask the neighbor who belongs to the offending feline to keep it off of your property. If that doesn't solve your dilemma, take some proactive steps of your own. Once cats find out that you mean business, they'll take their business elsewhere.
Use wire cutters to trim lengths of carpenter's cloth into 24-inch-wide strips. Spray them with eucalyptus oil or anise oil, which cats dislike. Lay the strips around your flower bed, creating a perimeter of harsh screening. Cats don't like the feel of the material on the soft pads of their feet. This is a double-whammy that may send them elsewhere.
Mulch small areas of the flower bed with pebbles. Cats don't care much for digging in rocks. They prefer spots where the soil is exposed and easy to work.
Combine 5 parts flour, 2 parts cayenne pepper and 3 parts dry mustard. Sprinkle it anywhere that you want to remain feline-free. Reapply as necessary, particularly after rainfall.
Tear open fresh or used tea bags and scatter the grounds throughout your flower bed. Cat's don't like tea leaves.
Repel cats from your flower bed by planting things they don't like. Put in some lemon thyme, lavender, geraniums and Coleus canina (scaredy cat plant). Intersperse these aromatic herb plants throughout the flower bed.
Set a molded child's kiddie pool up in an area near your flower bed and fill the bottom with 2 inches of cheap sand. Cats will most likely find this even more delightful than urinating in the loose earth of your garden. As they begin using it, move it a little further away from your flower bed each day until you've got it situated in a more acceptable area. Plant some catnip very close to the sandbox to give cats even more reason to prefer the designated cat area to your flower bed.
Flood areas that have been assaulted by urine with plenty of fresh water 3 days in a row. That should dilute the damaging salts that have been deposited in the soil and render them harmless to your plants.