This tutorial shows you how to easily kill / force quit an application or window that freezes or stops responding using xkill & shortcut key.
Xkill is a utility for forcing the X server to close connections to clients. This program is very dangerous, but is useful for aborting programs that have displayed undesired windows on a user’s screen. It works on Ubuntu, Linux Mint, Elementary OS, and other Linux systems.
You can run Xkill from terminal, Alt+F2 run command box or even via a keyboard shortcut. When it runs, your cursor becomes a cross (‘X’). Click on any window will kill it instantly.
Warning: Don’t click on Desktop and Panel when running xkill, it will also kill them and break your desktop and you need to restart to restore.
To create a shortcut key for xkill, go to System Settings -> Keyboard -> Shortcut -> Custom shortcuts. Click add to create the shortcut and give a key combination.
After that, press the key combination and then click on the window you want to kill.