I add an extra field to the form called "surname" and hide it using css.

I find that a bot will complete all the fields (including the hidden one). If my form processing script detects the surname field has been completed then the email is redirected to my spam mailbox which I only check a couple of times a month.

In the 6 months I've been using this method I haven't had any false positives so I'm actually thinking of just deleting the message rather than spamming it.