Add challenge to DH form mail?

I’ve started to get spam through my contact form. I’ve been using the DH form mail script for a couple years without problems, but I’m wondering if it’s possible to add some kind of image recognition or similar challenge field to discourage spam. I’m thinking of something simple like “enter my first name” or “enter the first word in the page title” etc.

I don’t think adding anything to the form is going to matter, since the spammers are bypassing the actual form and somehow posting around it.

I, too, am starting to get a lot of spam from my form. I might switch to another form processor, such as CGIemail, which uses a separate template file to format the message.