I created an app that gives the user 3 coins at the first launch.   

Once the user finished the 3 coins he should buy unlimited coins by an In-App purchase.  

The problem is that if the user doesn't want to buy unlimited coins he can delete the app, install the app again and he will have again the 3 coins again.  
And he can do this forever without ever buying the In-App purchase.  

Is there a simple way to prevent this? How can I know if a user finished the coins inside the app even after deleting it?