In order to protect your phpBB 2 forum from unwanted messages, you can use the Akismet mod for phpBB 2. This module is based on the Automattic Kismet (Akismet) project, which is dedicated to fighting spam.

To use the Akismet mod, you should first install the Spam Words mod for phpBB 2. Then you should download the spamwords.php.txt and functions_akismet.php.txt files, remove the .txt extensions and upload the files to your forum's directory > includes.

At the top of the functions_akismet.php file you should type your WordPress.com API  Key so that the mod can connect to Akismet and perform the necessary checks which determine whether a certain message is spam or not.

You can check and delete the spam messages from your phpBB 2 admin area > Flagged Posts.