Here's a few sites that may help a bit. Also Catsnclay has this problem and their Dr. prescribed some medication. That may be an option if you know which one is doing it.

http://www.all-creatures.org/aip/nl-12nov2002-cats.html

http://members.aol.com/ukaim/frame.htm?b_spraying.htm

http://www.fabcats.org/is10.html