This happened to me too.
1. Go to the Barnes & Noble website at www.BN.com and sign in to your account.
2. Click on "My Nook" towards the top right of the webpage.
3. You will see something that says "Nook Notification" in blue. This is found underneath the heading "Manage My NOOK" to the right side of the page.
4. Click on "Nook Notification." It should show the new friend message.
5. On your Nook click on the friend notification. It should stop coming back after this.