Use the BB gun, seriously. Fire a warning shot the second you see it (aim right behind it, hit the ground)
If it doesn't budge, BB gun. Go pro or go home.
If the cat comes back even after the BB, just call animal control, worst case scenario it costs you maybe $50 tops, depending on your area and how risky it is for the manager.
possibility:
Put a sleeping agent in the food? At that point you have a few options: take it to a vet and have it's back checked for a microchip (most pets have them,) call animal control for an easy cleanup, or just keep giving it the sleeping agent until it gets the picture.
ITT butthurt stray lovers
If a stray came near my dog and bullied her, I wouldn't give that stray a chance at all, I don't tolerate ANYONE or ANYTHING picking on my dog. I wouldn't bother asking the forum or bothering with a peaceful approach. Unless someone I knew owned the cat or it had a collar on... It would be a baaaad day for kitty.