dry mustard powder.
coffee grounds (ask for free grounds at your local cafe)
used tea leaves.
Nature's Wisdom Orange Oil Concentrate.
Shake-Away (cat repellant granules)
Fresh pine cones are sharp and cats don't like walking on them.
If the cats have owners, please contact them and explain your dilemma and ask them to take care of their cats.
If feral, then attach a motion activated sensor to a hose/spigot. Cats do not like to be wet, plus this is good for your garden. You can get this set up at a diy store.
You can use pecan shell mulch. The edges are sharp and cats don't like to walk or dig in them. Smells like peppers, moth balls, run them out.