The message boards portlet provides a built-in banning mechanism, which allows us to block a use from viewing and participating in forums.
To ban a user, perform these steps:
Log in as an administrator and go to the page on which the message board should be placed.
Go to the post added by the user you want to ban.
Click on the Ban This User link located under the screen name (or name and surname) of the user.
Click on the Banned Users tab in the message boards menu. The user you have just banned will be on the list.
To unban a user, perform these steps: