By sending them out manually by email instead of posting a download link. If you must post a download link (click bank or whatever) then you need to change it frequently. Name it something random (name your directory & thank you page this way too), and be sure you include a blank index.html in the directory that holds your ebook.
You may also be able to get it on the no share list here; contact one of the mods for info. Or, you could share it with us and gain some rep. Good luck!