I am most definitely a cat person, but I have no love for ferals (cat, dogs or people for that matter). I'd suggest the cayenne pepper to start with. Don't be stingy, either. You want a good amount of it to have the desired (and memorable) effect.
The cats won't damage your plants if they can help it, certainly not like rabbits (BIG problem animal here) and if you're washing your veg before you use it like you should be anyway their droppings are no different than manure fertiliser, but if it's a problem for you then you need to take action. Just try to look for collars on the critters before you plink them so you don't take out "Mr Snuffles" from the old lady's house down the road. REAL ferals make good fertiliser.