CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart.
It refers to small tests that usually accompany form submissions and account creation processes, to help reduce the impact of spambots and account-creation robots.
Captcha cannot be implemented without server-side support.
Webflow implements Captcha, but a slightly older version, which requires clicking a checkbox. The newer versions which are even more automated are not yet widely supported.
CAPTCHA will not block all spam. Spammers are creative, and motivated, and find ways around it.
For more extensive challenges, you can use hCaptcha to present better challenges to users. Good for account signups, for example.
See Jessica Galluzzo's work on this;