Instructions
Click on "Wii Options" in the lower left-hand corner of the Wii Menu and select "Data Management."
Select "Save Data" on the next screen. You can delete game saves and the Internet history from this page.
Click on an item and choose "Erase." The bottom of the screen will display the available save blocks after you delete these items. You can also delete files from any memory cards inserted into the console from this page.
Go next to the "Channels" page. This will display the channels that are on the Wii Menu. From this screen you can follow the same procedure in Step 3 to delete channels.
Return to the initial "Wii Options" page and select "Wii Settings."
Scroll to the third screen of the settings menu and select "Format Wii System Memory." Use this option only if you want to delete the entire Wii console history as this will delete all of the memory.