Okay no one really seems to be helping with your problem -- just encouraging you to let him keep coming in. I personally wouldn't like this either -- I think people aren't paying attention to the part about him scaring your cat and traumatizing her in her own home. You said he had an owner -- that should be enough. If he was a poor stray looking for a home, I'd say contact a no-kill shelter around you, but he has a home and a place to eat, so you made it sound.
When you say you have "water at the ready" does that mean that you have a waterbottle to squirt him when he comes in? This may or may not work -- chances are if he is bold enough to enter another's home then he is bold enough to not care about a little water spritz. I think that the thing to do is talk to his owners. I don't know where you live, but here there are laws against having your cats roam on other people's property. Here it is illegal for your cat to be off your property without a leash, and the way I understood it, it was like that in most states. I would check out the law where you are about this. If it's in fact, illegal -- then approach the owner of this cat and inform them of what's going on, and that it's against the law. If it's *not* against the law where you are, then I'd talk to the owner anyway and stress to them that you are not happy with this and want it to stop. It might be enough for them to get a running line for their cat so it could still be outside, but not enter other people's homes and eat their cat food. If none of this works, I'd try putting the catfood outside for him, and then eventually putting it further and further away from your home.
PS -- I hate this too -- it irritates me. I love cats to death and I want every one that I see, but my neighbor used to let all her cats outside without a leash, and they would come over into our yard and poop in our sandbox. This is so inconsiderate and rude of the owner -- cat poop can be fatal to kids and is obviously not something you want floating around in your yard where children play, or in their sandbox. My mom was too shy to talk to her so I did. Because it was illegal of her and I told her that we'd call animal control if she didn't keep better track of them.