I have an indoor/outdoor cat - how do I discourage other cats from coming into her territory?

There's a tom cat who belongs to a neighbor which comes over into our yard and sometimes chases our cat. Is there anyway to discourage the cat? Our cat is spayed, so I'm not worried about that, I am simply worried they might fight, since she came home a month ago with what looked like a scratch. And no, I don't plan on making her a true house cat - we live in a safe area for cats and cats, imo, are happier if they are allowed to venture outdoors.