Most spam bots search for forms, fill out every field and submit it. HoneyPot captcha will be hidden from store customers using css, however  spam bots usually do not load css and for them honeycaptcha field will be visible. If the honeypot captcha field is filled out then you know that it is a spam submission.
The extension provides honeypot captcha for magento forms.  The extnsion can be enabled or disabled separately for each of forms.
Check out extension at the Sashas Extensions website or at Magento ® Connect