First, bear in mind that we do not recommend disabling the thank-you email for download products, because the buyer may not realize they need to click a link to continue after completing checkout in order reach their download page, or the actual payment process may not have completed by the time the buyer finishes checkout, so in that case there would be no link presented right away even if they do click to continue.
The thank-you email provides a failsafe fallback to ensure the buyer can always obtain the download they paid for, as soon as their actual payment is complete. That said, you can disable the thank-you email by clicking Enable Templated Email in the product's settings but leaving the Email Message field blank (since our system cannot use a template to generate the thank-you email message if no template is provided).
