From nookDevs

If you register your Nook, and then wipe it w/o deregistering it.. and want to use a new account, you can end up in a catch-22 to get it re-registered. The simplest way to de-register a nook is on the Nook itself in Settings. However you can also deregister it at [www.bn.com]:

  - Login with your Nook email and password
  - Select account settings under 'My Account' in the top right hand side.
  - Select 'My Nook' in the 'Digital Management section'
  - Select the nook you want to de-register ( use the serial number to verify ) and click 'De-Register'.

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The above method does not work anymore. If you follow the instructions, after you click 'De-Register', a popup box displays instructions to de-register on the Nook Color itself or to call 1-800-THE-BOOK.