We really do not recommend sending buyers to a static download page on your own site, as buyers could share the URL of that page with others, and once it's loose "in the wild", then anybody can visit that page directly and download your product without paying you first. If you simply use our standard thank-you/download pages, those will issue each buyer a unique link that expires after the number of attempts and hours you specify in the product settings.
However, if you wish to proceed with your current setup, this page will be helpful:
http://www.e-junkie.com/ej/help.custom.thankyou-email.htm
To disable the product-specific thank-you email, just edit the product's settings in Admin, check Enable Templated Email for that product and leave the Email Message field above that blank, then click Next until you can Submit changes.