I made it so that "Guests" can no longer make bookshelves. You now need to have an account and log in. It's free and easy so I don't think that it's too much to ask. Also bookshelves are now 'public' by default. If you want your bookshelf to be 'private' (say, its a gift list for someone and you don't want them to know about it!) You need to set it to 'private' on in 'Bookshelf Manager'.