How do you prevent players from accessing any scene they want by just renaming level0, level1, level2 etc..?

I'm developing a game and I've just created a third level which is DLC and has to be paid for to be unlocked. To make the process of unlocking and playing this level as simple as possible it is included with the standard game, and then unlocked by entering a code in game.

I've noticed that in my data folder I have the files level0, level1, level2 corresponding to my scenes (although there's a 4th very small scene which doesn't seem to have a corresponding levelX file)

Unfortunately all I have to do to access the DLC scene is swap its name with another scene which is unlocked by default. Then I select that level as normal in the game, and the DLC one loads up and works 100%.

Obviously I need to prevent this from working, or obfuscate the files so it's not an obvious thing to try.

I could perform the same "authorisation" check in the DLC scene that I perform in the menu to make sure it's unlocked, and this will probably be enough, but I'd still like to know how to prevent players from doing this little trick.

Thanks!