Best way to create a kill switch?

What would be the best way to implement a kill switch to disable an app if it is stolen, or something?

For example, you have a file that the app checks to see if a condition is 0 or 1. If it is 0, it goes as normal. If the file variable is changed to 1, a message appears saying that the app is disabled.

Anyone have experience with something like this?

What would stop them from copying the file itself?

damjancd This was mostly for in case a client doesn't pay. The stolen part was just an example

Update:

I went with ajax using this tutorial: https://www.scirra.com/tutorials/61/aja ... onstruct-2

It simply checks a php file on my server at start of layout for its contents. The file will contain either a 1 or 0. In contruct it is programmed to disable everything and only show a message to payup if the file contains a 1. I can easily change it to 1 or 0 by editing the php file