Instructions
Turn on your Nintendo Wii console, but do not load your "Wii Fit" disc.
Locate the "Wii Fit Channel" box on your Wii home screen. Click it and wait for it to load.
Select the Mii that you want to delete from the ones displayed on the main screen of the "Wii Fit Channel." Click "Begin" in the top-right corner of the screen.
Point at and click "User Settings" in the individual Mii's home screen, and then choose "Delete User Data." The Mii will no longer appear in "Wii Fit."