I don't see the field you're referring to, but I'll try to answer with a best guess anyway.
Most web spam bots fall into two categories:
1. They target a certain web software package (such as WordPress), for which they know the HTML layout and the range of possible CAPTCHA challenges.
2. They try to fill out every HTML form they find blindly, using heuristics based on the names of form input elements.
To defeat both, all you need to do is to write a custom HTML form by hand (which defeats category 1), and add a simple CAPTCHA challenge (which defeats category 2).
Thanks. But that challenge is so trivial to defeat by simple pattern matching with zero cost that the cost of adding code to a bot to break this type of challenge seems negligible.
PS: it's a the bottom of the landing page (not signup)
Most web spam bots fall into two categories:
1. They target a certain web software package (such as WordPress), for which they know the HTML layout and the range of possible CAPTCHA challenges.
2. They try to fill out every HTML form they find blindly, using heuristics based on the names of form input elements.
To defeat both, all you need to do is to write a custom HTML form by hand (which defeats category 1), and add a simple CAPTCHA challenge (which defeats category 2).