You can use charcoal briquettes (the kind you barbecue with), but NOT the self-igniting kind! (Of course, you knew that, right?) You can sort of crush them up a little bit (you still want them kind of chunky) and then spread them over the surface of the crawlspace floor. You'll probably find that the cats used a particular area most frequently for their business, so get a good layer over that.
If that is not totally successful, you can try spraying the space with good old white vinegar (right over the charcoal). That, of course, will make your house smell of vinegar for short while, but that would probably be better than cat p - - -, huh?! Good luck!