Instructions
Turn on your Wii console and your Wii remote. Insert the "Wii Fit" disc. Wait for the console to read the disc and click "Play Wii Fit." The "Wii Fit" plaza will appear when the game loads. All of the Miis on the game will appear, too.
Click the Mii you wish to delete. Click "User Settings" at the top of the screen.
Click "Delete User Data." A pop-up box will open and ask you if you're sure. Select "Yes." The Mii is now deleted.