Basically, you can't - not in practice. You can make it harder, but you can't stop it completely. The big companies spend a fortune to try and do this, but even so ... Adobe protect Photoshop pretty well, but the same day as a new version is released, the cracked version is up on torrent sites.
Don't spend too much time on this, or it will cost you more than you will save in "lost orders". But have a look at google:
prevent copying my application c# - Google Search[
^]