As said above by XFlak, it is impossible to remove all the traces Nintendo can find (and looks for), so there is no real reason in trying, except stuff like tricking other people.
EDIT: As someone pointed out to me, there are 2 ways that should make it possible to 'virginize' the Wii, but method 1 requires a NAND backup before you did anything except installing bootMii into boot2, an old boot1 to be able to do so and boot2 must be older than v4. And method 2 requires a NAND programmer or something similar. And if you know how to use and have such stuff you should also be able to repair you Wii, save for spare parts.