Yeah, gonna have to agree that these cats being strays is the real problem here. Any number of health concerns come to mind. As for cat repellents... Nnn, I can't remember any that actually work, except for motion sensor activated squirt guns and such like I saw on youtube. The Mythbusters managed to debunk a bunch of the ones people talk about like the water bottle thing, but I don't remember them finding one that worked to just keep them from wanting to go somewhere. You gotta figure something would do it, like a particular scent.