How to Delete a Locked Mii on "Wii Fit"
When you make a Mii and add it to "Wii Fit," you are given the option to password protect the character so that the information isn't accessible to other people on the console. However, sometimes a friend makes the Mii and then leaves, with a password-protected Mii still on your "Wii Fit" or "Wii Fit Plus" game.
There is a way to delete that Mii from your console, but you do need to know a little about the person who made the avatar.
Power on your Wii console and begin the "Wii Fit" or "Wii Fit Plus" game. Select the Mii you intend to delete.
Attempt to break the password. If you know the person well, type in what you think their password might be. People often use important dates as their password, such as birthdays or anniversaries.
Make three to five guesses. If they are all wrong, the console will ask if you really are the player. If you manage to break the password, skip the next two steps.
Input the original player's height, in inches, when prompted by the console.
Select "Delete User Data" from the "User settings" screen. The system will erase the Mii from your console.
Return to the main Wii menu. You can now enter the "Mii Plaza" channel and delete the Mii from there if you would like to do so.