I did a bit of research on this and found that cats hate the scent of citrus. I tested it out — bought a few lemons and made lemonade (why not?) and showed the peel to my cats... they scampered off immediately. So I bought a few of those citrus air fresheners and placed them around the house. Here's to hoping it works.
By the way, while doing the research, I read that spraying a cat with water to discipline him isn't recommended because it actually has the opposite effect: it antagonizes him. Also, they will supposedly start to associate you with the water bottle and general unpleasantness. I gotta say that that was rather surprising since I've heard many people also recommend the spray bottle technique. Hmm...