You can do this on the Generation Settings page for your book, which you can find by going here:
...making sure to replace YOUR_BOOK with your book's unique web address.
you can modify whether we generate no web page, a sample HTML version or a full HTML version. So, if you choose "No web page" you should get what you want.
Please note that if you choose "No web page" we still allow Leanpub readers who have bought the book to read the EPUB in their web browser. EPUB is just zipped HTML plus some metadata, of course. However, we assume that's OK :)