You can configure OJS to use CAPTCHA challenges to prevent user registration and comment abuse. Find the CAPTCHA Settings section in
config.inc.php and uncomment and change to the following:
[captcha] ; Whether or not to enable Captcha features captcha = On ; Whether or not to use Captcha on user registration captcha_on_register = On ; Whether or not to use Captcha on user comments captcha_on_comments = On ; Font location for font to use in Captcha images font_location = /usr/share/fonts/truetype/freefont/FreeSerif.ttf
Please note that your server needs to support the GD (PHP4) or GD2 (PHP5) library for this to work, and that you need to point your
font_location variable to a font that exists on your system, which may be different than the default provided.
Repost from: here.