I ended up doing a very basic "captcha" style implementation on my contact form - just to get the user to type in a word, in a certain case, otherwise the request fails validation - http://www.zazzybob.com/contact.html - this is an *extremely* basic form of validation - but it works - and I no longer get my daily Viagra and Rolex ads ;-)

Something like this may be more robust....

Cheers
ZB