That suggestion you have about the input form has merit. In fact, you don't even need to put a value in it. Just leave it blank by default, and it if gets filled in, then abort the registration. A few months ago, I ran a honeypot board which had 5 hidden form fields. They were in a

tag marked as hidden, so the only way to see them was to view the page source code. Every bot which signed up filled out these hidden fields.