If you go to https://leanpub.com/YOURBOOK/edit_generation_settings and scroll to the bottom, you'll see the following:
DISPLAY YOUR BOOK AS A WEB PAGE ON LEANPUB?
No web page
Just the sample
The full book
This feature lets you generate a web page of your entire book (or just the sample) and put a link to it on your book's landing page.
To turn it off, just choose "No web page".