EDIT: dumb mobile site and not taking me to the last page when I think it does. Makes me answer questions that have already been answered.
EDIT 2:
@JustApollo and Spansh: While formatting will work on a hacked wii, it won't get rid of all your modifications. I don't believe it touches any IOSs or the system menu, so things such as priiloader and bootmii and cIOSs won't be removed, but any channels you had will be, this will also delete all your legit stuff to, like saves and things downloaded from the wii shop.
Instead, if you want to remove stuff, I would start by using something like MMM to remove any IOS above 200. Then uninstall any channels you installed using WADs, and remove the homebrew channel (just through data management. You could use AnyTitleDeleter, but that could get really dangerous if you don know what you are doing). Next turn on your wii while holding the reset button. If priiloader comes up then go to system menu hacks and make sure they are all disabled. Next go to the wii menu, then to the system settings, and install a wii update. Finally, remove all the homebrew stuff on your SD card.
That's about it, that is as close to clean as you will get (well, you can get rid of more, but then you run a higher risk of bricking your wii).