You can only make it more difficult to access the file, not make it impossible. One step you could take could be to encrypt the file in some fashion before it is placed in the IPA. Sure, the user could root out the key and decrypt it anyway, but it would stop the more casual users who are trying to snoop.
The prevailing theory is that it's much better to spend time investing in the software by adding features, enhancements, etc., for the paying users rather than spending time trying to stop a few users from misusing the software. You'll end up with a higher quality product that will perform better on the market.