Once determined spammers know that the hidden field is there, they tell their bots not to fill it in.
The best spam-proofing by far is a math equation.
If it's a simple contact form, check out SPForm (with the mathstring option).
If not, you can implement the MODX Captcha plugin for any form. You can look at the SPForm code to see how it's done - set Captcha's mathstring system setting.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html