Instructions
Turn your Wii on. Select the "Wii Shop Channel" option in the upper right corner of the main menu. Select the "Settings" option in the shop channel main menu, then select the "Remove Wii Shop Channel Account" option. After the account has been removed, return to the main menu.
Access the Wii settings menu by selecting the "Wii" icon at the bottom left corner of the Wii main menu. Select the "Wii System Settings" option in the Wii settings menu.
Select "Format Wii System Memory" in the system settings menu, and select the "Format" option to reset the Wii's settings.