It should be possible fairly easily by editing the 'contact' template. I can write a quick example if you like. That said, anyone interested in spamming you would completely ignore the message, which sort of beats the point. Are you using a CAPTCHA (be it Q&A captcha or reCAPTCHA) on the form? That's a good spam deterrent.
EDIT: Hah, ninja'd Brogan for once instead of the other way around. Yay me.
I am using an unusual anti-spam question, which has worked well. My message is concerned with in particular human spammer that advertises "forum services," and is aimed not so much as stopping him as to give something to point out when replying to him.