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.