The only thing I can think might help is a citrus spray. Apply to your porch and anything near the door as often as you can. Maybe this will encourage the outside cat to go somewhere else. Or possibly a couple of moth balls in an old nylon (anchored) could be left on the porch for a couple of days. Cats don't like the smell (who does?) and they won't be able to move it. You'd have to endure the odor until the cat stopped coming around, but it should work.