After purchasing a Kirby license you will receive your unique license code.
site/config/config.php and search for
c::set('license', 'your license key');
Fill in your license code to complete your installation of Kirby.
There's no validity check of your code or any other communication between your Kirby installation and my server. You get a clean installation without any hacks or tricks. Entering the license code is for your own records, so you can track which installation uses which license.
I'm focusing on making Kirby better instead of fighting software pirates. I trust in you to support me with a legally purchased license if you like Kirby.
You can read more about this here.