After you've registered and signed in with your Yahoo! ID, go to the "Message Boards Settings" page by clicking the Message Boards Settings link. It should be located in the upper-right corner of every page on Message Boards. You are automatically sent into the Profanity Settings section.

You can choose to turn your profanity settings On or Off, select the strength of your filter, and whether you want to strikethrough profanity or hide the entire message.

Click the Save button to have your changes take effect and return to the message boards.