Previous versions of Joomla (notably 1.0.x, 1.5.x, and 1.7) required multiple steps to implement spam protection on your Joomla site. To enable this protection on the Registration or Contact pages, you had to download, install and configure a captcha plugin. Beginning with Joomla 2.5, you can quickly enable Captcha (ReCaptcha) from within the default Joomla installation!
First of all, you have to get public and private keys from recaptcha.net and generate it. This is a free service, but you will need a Google account to create captcha keys for your main domain. Now you are almost ready to protect your Joomla Web forms from spam bots.
From the Joomla Admin menu, select Extensions->Plugin Manager. On the plugin list, locate and select ‘Captcha-ReCaptcha’ to open it, paste your public/private keys into it, then set status to ‘Enabled’. Finally, save your changes. (By the way, you can also specify different themes for the captcha, but the ‘clean’ style is probably best for most needs.)
To protect your contact form, go to Site->Global Configuration->Site (tab) and from the Default Captcha list, select Captcha – ReCaptcha.
If you want to add a captcha to protect your website user registration fields, simply go to Users->User Manager and click the Options icon on the right side. From the Captcha list choose ‘Recaptcha plugin’ then save changes. That’s all! The Recaptcha security images will also work with Joomla’s reset password and remind username options!
Now you and your clients can stop worrying about form spammers!