I feel like creating a honey pot for bad bots now. Put an exclude line in ROBOTS.TXT and then include that URL in my pages and when a bot hits it anyways, ban the IP.
FWIW I've been using robots.txt honeypots on two sites for the past few years to deter scrapers. Approx 1,600 IP addresses have been banned. Most of these are for data centers.
I'm not alone :-) see http://incredibill.blogspot.com/
IIRC, technically you can have a 0 pixel image in some formats (gif) though I imagine some browsers won't like that.