I just wanted to chime in on this because I get a massive hot break and I've done most of the things mentioned here to try to control boil-over (with the exception of adding olive oil--which is a really bad idea, BTW, for the reason mentioned above). I've sprayed water, backed off on the boil, hit the kettle with a fan, added Fermcap. I've found that the easiest, quickest, least annoying/disruptive of all alternatives--and one that doesn't involve adding anything to the beer, for you purists out there (of which I'm becoming one, incidentally)--is simply skimming the hot break, as a few people have mentioned here. You can maintain your boil, you don't have to stand watching, you don't have to add anything you may or may not trust. Do it and you're done. 'Takes about 30 seconds. You can ignore the kettle from then on and go about cleaning and sanitizing. I realize that people like their own systems, and they like to stick to techniques they're familiar with, but this is a time-saver and a stress-reliever. I'd recommend giving it a try.
Oh, and there's no effect on head retention in the finished beer, in case that's a concern. At least, I haven't found there to be....
