We get some spammy user registrations, but not so many that this is an urgent problem. Phabricator upstream has recaptcha support, but recaptcha seems privacy invasive (not only detecting humans, but which particular individuals), among other problems (not free software, etc).
But it would be easy to add a simple captcha that can eliminate non-targeting "drive-by" spammers. So we might do that.
Or implement this: https://secure.phabricator.com/T12643