How do I get a stray cat to go away?

A stray cat (it has no tags or collar) has been coming to my windows and haring my cats for two weeks now. I shooed it off when it was at the first floor window, but now it climbs up a little roof and sits in the second story windows several times a day. Knocking on the closed window and hollering only makes it hiss at me. I'm worried if I open the window and shoo it off it will try to hurt me or dart in the house. My cats are seriously freaking out because of it; one attacked me shortly after the stray started coming around! He's never done anything like that before! I haven't been able to open my windows for fear the cats will go through the screens trying to fight. I don't want to hurt the stray, so no pigeon spikes or anything, but it has to stop coming around! It may be someone's pet, so I am hesitant to have it trapped. It climbs onto a neighbor's roof to get on mine, so I can't block its way up. What can I do to make it not want to come up there?