I tripped across this old message in some archives and thought it was a unique addition to the choices on the Contact Us page that may offer some more defense:
Undertoad Jul 3rd 2006, 4:20pm
Here's how I deal with it, via "Contact Us Options":
Even with image verification off, this simple method worked perfectly.
No spammer ever figured out that they should change the default setting. Every legitimate contact us user changed the setting.
Gmail's spam filtering picked up only the spam messages. Problem solved.
above taken from
http://www.vbulletin.com/forum/archi.../t-188024.html