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 now 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 put your changes into effect and return to the message boards.