Instructions
Removal From Wii Fit
Turn on the Wii console and television, then insert the Wii Fit game disc into the console.
Point at the Wii Fit game on the upper left of the screen and select it when the game icon emerges from the spinning disk. Select "Start" from the next screen. You'll land in the Wii Plaza, where all your registered Miis will be looking at you.
Choose the Mii you want to be rid of and enter the Mii's password if you're asked for it. You will land at the Calendar Screen. The doomed Mii will be standing on the right next to a giant calendar. Above its head will be a round icon that represents user settings.
Click the icon for user settings.
Choose the last of the four options presented. It will say, "Delete User Data." When the Wii asks if you are sure, confirm that you are.
Removal From the Wii Fit Channel
Visit the Wii Fit Channel on the Wii Menu if you initially created one when you set up the game.
Choose the appropriate Mii from within the Wii Plaza, providing a password if asked.
Click the user settings icon above the Mii's head. Choose the "Delete User Data" option on the bottom and confirm that you want to delete the Mii's data.