Hi there. My name is*****'m sorry that this happened to you. It would be my pleasure to help. The first step is to login to the Barnes and Noble website at https://www.barnesandnoble.com/account/manage/settings/ and scroll down to the "Change Your Password" section. Once you change this no one will be able to purchase anything using any passwords that may be stored on your Nook. After you've done that, if your Nook allowed you to make purchases from the Google Play store, you will also need to change your Google account password. Please let me know if that is the case, and I will provide detailed instructions if you need to that as well.
To be absolutely sure that no one can purchase anything using your Nook, please call Nook Customer Service toll-free at 1-800-THE-BOOK(###) ###-#### toreport your Nook as stolen and to unregister the device from your BN.com account. Not only will this keep them from being able to purchase anything on your account, but it may also prevent them from being able to format the Nook and use it themselves under another account.
Please give this a try and let me know how it works for you. If this solves the problem, would you mind taking a moment to rate my service? You can give your rating by selecting the stars located on the top right of the question page. I'd really appreciate it! Without a positive rating I am not credited for my answer. If this doesn't solve the problem be sure to let me know so we can continue to work on this.