The best approach is not to give it to users you don't trust. If you are afraid they will share it then get new testers.

It would probably be worth mentioning that M$ tries this with XP and yes it isn't difficult to find the hack online.

If you insist on trying it then i would consider a two step approach. Build a utility that they run and it gives them a signature of there system. Then build your exe that uses the signature to limit the use. You could probably use the signature in some sort of encryption to make sure the EXE only runs if it gets the right key.

If I remember correctly anyone can just rename your .exe .zip and then open it up and see the source code (perhaps thats a different 2exe utility though.)

Whatever you do rememeber it is just a deterant and there is no fail safe way to accomplish your goal.