You Cannot Close The Windows Client
ALT+F4 normally closes an application on Windows. You have decided to override this so that it doesn't work.
Worse than that, if you right click on the application icon in the Windows taskbar and click Close window that also doesn't work and the application freezes.
Even worse than that, if you are in the game and click on Settings > Exit Game not even that works, it just freezes the game. Your own intended method of closing the application doesn't even work...
Do you think that if you somehow prevent me from closing the application that I will decide to play more or spend money? Just close the application and stop trying to be clever and ******* off your users in the process.
To close the application I typically have to press Ctrl+Alt+Delete and kill MTGA.exe by hand. You should be embarrassed by this. I have 12 years of professional software development, and this should be fixable by someone with 1 month of coding experience. A coder that knows how to press Ctrl+F and search the source code could probably fix this on their first day!
In this regard, your client operates more like malware than an actual legitimate application.