In order to protect your Joomla application from unwanted messages, you can use the Akismet component for Joomla (there are versions for both Joomla 1.0.x and 1.5.x). This component is based on the Automattic Kismet (Akismet) project, which is dedicated to fighting spam.

The component is installed as any other Joomla component. Once you install Akismet4Joomla, you can configure it from your Joomla admin area > Components > Akismet. In order to use Akismet, you should enter your WordPress.com API  Key in the corresponding field.