You should be able to erase and deregister remotely via http://mynook.com.
If you don't have a mynook.com account you'll have to set one up - I think if you use your same email and password as your B&N account and it will automatically link it to your B&N account. Then you select "Manage My Nook". When you scroll over the nook you want to deregister, a popup will appear with a link to erase and dereigster the nook.
Your nook will need to have wifi turned on to do this. Can you see the part of the nook screen that will let you turn on the wifi?