Preventing demo executables from running final build

Hi all,

We're developing a game and recently created a demo with only a few levels included. We realized we could play the full game with the demo exe by swapping the executables since they are identical.

How can we prevent users from just swapping in the demo exe with the full game's data folder and playing for free?

If this has been asked before I'm sorry, I couldn't find anything directly relating to this.