When you look at the live/published view of a Hub, the button to delete the entire thing is right at the top, in between the stats button and the word "tools."
Once you hit delete, the Hub will show on your stats page with a line through the name for 24 hours, in case you change your mind. After a day, it will be permanently deleted and will disappear.
NOTE: Although people keep saying that a delete option appears in the edit function, I don't see one there when I view my Hubs that way. I only see "preview," "unpublish now" and "done editing" buttons.