Deleted Comment
Long time ago I saw a post where someone running a blog was having trouble keeping spam out of their comments, and eventually had this same idea. The spambots just filled out every form field they could, so he added a checkbox, hid the checkbox with CSS, and rejected any submission that included it. At least at the time it worked far better than anything else they'd tried.
1. https://myaccount.google.com/connections/settings for mobile
2. same link, sidebar help, for Chrome on Desktop settings.
https://github.com/samyk/pwnat
It has more edges and doesn't handle all cases, but it also avoids the need for any kind of intermediary.