What volume of spam are you receiving?
It's interesting that you should say that. Honestly, I never really get reports of that. I do get the occasional spam email through my own installations on my websites, but spam unfortunately is an ongoing issue. There are no applications or services that are even close to 100% spam-proof with no false-positives. I get hundreds of spam submissions to my blogs and forum a week.
I assume you're referring to spam submissions on your form and not spam email in general? For instance putting your email address on your website will attract a lot of spam, but a lot of people recommend doing it anyway because users often prefer to easily find an email address.
I also dislike CAPTCHAs, partly because they're ineffective but mainly because they make spam the user's problem rather than the website owner's. I'd rather make life easier on my users and get a bit of spam.
I do have plans to add exactly the feature you describe - an anti-spam question field type - but I don't have a timeframe for it, because it's not a high-demand feature. I'm afraid it's too complex a feature to write a hack for on my forum.