I remember someone a few years ago telling me about a simple behaviour they'd setup where a hidden checkbox (visibility:hidden) was an option in their form (ie., E-Newsletter signup), and that only spambots could/would check the box. Then something setup in the backend (maybe javascript, I don't remember) basically followed the instruction of "if box is checked, then this is spam, so don't record the entry, ignore it".

I'd love to set something up on like this on a couple of sites, but not sure how to go about it. Any advice would help,

This is possible with the default CAPTCHA Antispam field, by setting the "Image Generation" property to "Invisible" from the Additional tab.

In this implementation a regular textbox is used, that is invisible to the user, but may still be filled in by a spambot, in which case the submission will be discarded as invalid.