Not the main/default NB site, just a second or third site created within the NB dashboard.
Within your Control Panel, navigate to Websites > Site Settings > Basic for the specific site you'd like to delete. In the bottom right corner of this page, you should see a 'Delete Site' button. Press the button and confirm the pop-up box, and the site will be deleted.
Note: You must always have at least one site. If you'd like to delete your nation's only site, you'll need to first create a new one.
Additionally you cannot delete your primary site - to check your primary site go to settings > basics > primary site drop down.