I think I used to do something similar with email spam traps. Not sure if it's still around but basically you could help build NaCL lists by posting an email address on your website somewhere that was visible in the source code but not visible to normal users, like in a div that was way on the left side of the screen.
Anyway, spammers that do regular expression searches for email addresses would email it and get their IPs added to naughty lists.