Try changing the country setting i.e. from the third page of the Wii settings menu. This worked for me - the PIN and all parental control settings were removed.
Background
I bought my Wii second hand and when I tried to access the internet settings, the PIN was requested.
I asked the previous owner and they said they hadn't set up a pin.
I eventually guessed the PIN, which was quite straight forward, but it was obvious from the 'mother's maiden name' answer that their kids had set this up.
After this I went on the weather channel and it thought I was in Ireland. Turns out the kids must have set this wrong too! But when I set this back to UK, a message appeared along the lines of 'resetting all parental controls' etc.
Hey presto - no longer a PIN.
Hope this works for someone else...