We're sorry, our documentation is a bit unclear here.
The "No web page" option means that your book is not available for free to read on the web.
If your book were available to read for free, you would see a box like this to the left of your book cover, on your book's landing page:
All Leanpub books published using one of our book writing modes is automatically available to everyone who purchases the book in PDF, EPUB, and MOBI, and on the web, and there is no way to disable any single format.
Please note that in order to read the "Web" version of your book, someone does have to buy the book.