To protect your Joomla application from unwanted messages, we recommend the Akismet component for Joomla (there are versions for Joomla 1.0.x and 1.5.x). This component is based on the Automattic Kismet (Akismet) project, which is one of the leading spam-fighting projects in the world.
The component is installed as a regular Joomla component depending on your version. Once you install Akismet4Joomla, you can configure it from your admin area .
First, you should go to the Components menu and select Akismet. To activate the component you must enter your WordPress.com API Key in the corresponding field. Once your do that, Akismet will take care to block all spam messages coming to your site.