@jackyq2015 the person who would be using your app would know they would pay you a fee on each transaction. If they keep using your app then they are fine with it and your app will automatically send a transaction to your account.
Of course the person can manually control the page's JavaScript to change that, or they can fork the project, or do a bunch of things, or -- much better -- they could just use the Stellar API directly to operate on the distributed exchange -- but all that would defeat the purpose of your app, that is to offer greater value on the UI.
So, in other words, anyone who's willing to dig into your app's JavaScript code just to change that will just not use your app in the first place.