A dude named Scott Wiersdorf has written a script to harrass formmail trollers when they hit his site. It's a fun idea, but I think it actually would cause me more harm than good. That's my bandwidth they're using!
Still, it makes me wonder if there's something I could do to get back at referrer spammers. Is there something that would consume very little system resources but still take a lot of their bot's time or other resources?
I suspect that even if it were possible, it would not be worth the effort.