You can make both the minimum and the suggested prices for a Leanpub book free, but people can still choose to pay if the want to; there's no way to absolutely prevent someone from paying for a book.
However, if you really don't want to make any money from your book, you can share 100% of your royalties with one of the organizations in our Leanpub for Causes program:
After you create a Leanpub book, you can add a cause by going here:
...making sure to replace YOUR_BOOK with your book's unique web address.