I’ve noticed spammers tend to try to fill out your whole form before trying to submit their spam. A while ago I tried adding a simple radio button field to the form with the value “this is probably spam”, and then hiding it with CSS.

The spam robots helpfully check the box for you before submitting, and you can set an email filter to trash all all the responses which have it checked.

Remember to label it appropriately so your users with CSS off know what to do with it ;)