Disabling a feature in ipa file

I have an ipa file for which I don't have the source code. This is just a regular ipa with developer signed. I would like to disable some feature in that file, repackage it and run it on a non-jail broken iPhone. I understand I can do this in jail broken device by writing a dynamic library and using mobile substrate. But how to do this in non jail broken device? Can someone throw some light on this?

Thanks in advance !!!