You can also go to the folder "assets", under "system", delete all contents then chmod 444.
That folder has all the special offers. The Kindle turns the permissions back to the default, but since I don't usually turn on the wifi, I only have to deal with the kindle asking me to go online to see the special offers.
Amazon Charged me $30 to get the Adverts Off my K4 http://www.the-digital-reader.com/2011/10/05/amazon-charged-...