Kelly touched on the subject of a poor man's CAPTCHA system in XMod. His ideas were spot on for great deterrents, but I wanted to list a few more options that may also work, depending on your user base.
 
The first option would be to create a regular expression that would validate primary colors or domestic auto manufacturers. Something like (red|yellow|blue) or (Dodge|Chevy|Ford). You would then have a text box as the last entry on the form asking the appropriate question with data validation turned on to use the regular expression you created. You could use any type of listing like these that may appeal to your site user base (ie: Car manufacturers on a car site).
 
I have seen some site owners use a simple question such as "Are you a human?" with data validation to only accept "yes" answers. Another way to trip up bots is to Ask a question such as "Which one of these items is a mammal (goat, dandelion, horseshoe)?" teamed with data validation to only accept "goat".
 
Another method that may work in certain scenarios is by checking the user agent. (Granted that you are ok limiting your site to the major browsers...) Most bots will show up in your referrer log differently than normal browsers. You could have a text box populated via an xmod constant (User Agent) through the use of a variable (I think this is possible in current versions of xmod) and then have a regular expression check for and allow common agents [0-9a-zA-Z_]*(gecko|IE|mozilla|explorer|firefox)[0-9a-zA-Z_]*
 
You could also do something similar with check the URL referrer. (This would only apply if the bot was hitting the form entry page dead on, which more than likely it would be, and then the referrer should be blank.) The regular expression to check that the referrer is from your site would be something like this: http://([w*.*yourdomain.com/*]+)
 
Some of these expressions listed above may need tweaking, I just threw them out there off the top of my head...
 
Happy XModding!