Currently, there is no way to disable the table of contents from showing up on a Leanpub book landing page.
We have learned over the years that showing the table of contents is really important for book sales and for inspiring confidence in Leanpub readers. It's a little bit like the book sample; it's kind of the equivalent of letting a customer in a conventional book store take the book down off the shelf and get a glance at what's inside.