Although this method is so simple everybody knows it but i got crowned and became the king of the day the day i assisted someone in force closing a stubborn app on her pc using this simple method so i thought of sharing it,
ITs simple all you need is the default task manager.
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[*]You can start your pc's task manager by pressing ctrl + shift + esc key consecutively on your keyboard.
[*]locate the stubborn app right click on it and click on goto process.
[*]right click again and click on end task.
[*]thats all!.

In case you do you not like the default windows task manager and you feel like a pro you can check here for best alternatives.
5 Powerful Alternatives To The Windows Task Manager

Some apps by default do not give users the opportunity to close or end them,using a task manager can help you end its process which will free up some RAM and you enjoy your pc experience hehehe,

just locate the app right click and end task or end process thats all

Hope it helped? please let us know.