[next89]

I think that it would be easily fixable. Every time you disable an account you send an email to the email address they registered letting them now about it with a link to reactivate it. For the reactivation, you can even add a normal catpcha to prevent the bots exploiting the loophole.

[scott_karana]

Kinda negates the UI advantage of this approach, doesn't it?

Maybe there's an alternative, like a Javascript-filled ARIA-field that would read out "do not fill this field: robot use only"?

[samuellevy]

I just label my fields appropriately, and put a "skip to form" link (like a "skip to content" link). Other options exist like using media detection, or JavaScript (very few bots run JavaScript on pages. It slows down the process, and requires more smarts than a generic html parser)

No negative captcha will stand up to a targeted attack, but if you're not running a huge site, targeted attacks are very unlikely.