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Apply Online CAPTCHA is an effective security solution that adds captcha challenge at the bottom of the application form. Either use industry level reCAPTCHA by Google which requires an API key from Google or Really Simple Captcha for quick installation.
reCAPTCHA by Google is a free service that protects your site from spam and abuse. It uses advanced risk analysis techniques to tell humans and bots apart.
Applicants are required to pass captcha challenge to confirm that they are not a robot before a form can be submitted.