I agree with the three S rule myself but if you're against either trapping it or shooting it to kill then get yourself a BB gun... load it and sit outside when you know he's around.. then shoot him with it and I guarantee you he'll take off. The pet store sprays and other stuff like that work ok but for an unfixed male cat the best thing to do is make the area as unpleasant as possible for him to be there.
I know this from experience as I had an almost 6 month war with a local tom cat that visited from a neighbor's farm... they had well over 100 cats!! I shot a few but got sick of doing that because they would just get replaced by another... and this one was quick enough to get outta the way before I could fire... I also used fire crackers and anything else that would make noise to scare him off. Just remember though, you'll have to keep it up to make him change his mind.
Good luck with this... I don't think there's any definite way to keep him away though without taking the two routes that others have suggested.