What can I do to stop other users working with my RT RT code?

Applications downloaded from the Windows repository are installed at this location:

C:\Program Files\WindowsApps

If you look into this folder, you can access each .exe application and use the reflector to decompile them.

Currently, the Windows RT application sends the password via SSL to the WCF service to ensure that only users using my application can access my database (through the service).

If my code can be read by anyone, how can I make sure that only users using my Windows 8 application access the service?

Thank!