Pligg supports some CAPTCHA, including Solve Media, and old reCAPTCHA.
Now i will write how to use reCAPTCHA v2 (click captcha) into Pligg.
First, you must get Google reCAPTCHA keys.
Second, open your Pligg admin, set the site & secret key under “CAPTCHA” sub menu and click “configure” next to reCAPTCHA.
Third, activate “reCAPTCHA” as CAPTCHA selection.
Fourth, upload these files & overwrite existing files :
/modules/captcha/captchas/reCaptcha/main.php http://pastebin.com/XWYp6tcN
and
/modules/captcha/captchas/reCaptcha/captcha.tpl http://pastebin.com/0FjHf2a9
Fifth, from your Pligg admin, clear the template cache.
Now your site should use reCAPTCHA v2.