AI - Artificial intelligence

I'd say, every time, generate a new page with some variations:

• randomly change the position of the checkbox
• randomly change the contrast colour of the checkbox as long as it is visible enough to humans (sorry accessibility)
• randomly change HTML tags (id, class etc) to prevent finding the check box by parsing source
• add trap checkboxes with display: none
• add "I am a robot" check boxes at other locations on a page
  • write them before the actual checkbox in HTML (to catch parsers), but use CSS to display it such that the user looking at the page will see the correct one more conspicuously
• write "I am a human" next to the trap checkbox in HTML, but use CSS to position it next to the correct one (this might be harder to implement properly)

That way, if one is to use a bot to click it, they will have to use machine vision, which will be expensive enough and satisfy Redex's condition.