Hiding download links

What's the best thing to use to hide the download link for a product. I've heard of stuff that personalises the 'Thank You' email and directs to a unique directory. But, presumably, that must create the directory and put the product in it as well.

Seems a bit confusing. But if I'm making the effort to flog something there I don't want lots of free downloads of it (I'm quite capable of sharing here by myself, thanks).

Any suggestions?