Sometimes, an application refuses to quit on Ubuntu and it becomes non-responding, which will oblige you to restart your computer. Fortunately, there is a command line utility called xkill that allows to kill any unresponsive application. The xkill command allows to force close a window or application with your graphical mouse pointer.
Open the terminal and issue this command:
Your mouse pointer will change to a cross, click the unresponsive program or window to force close it.
You can also run xkill by pressing ALT + F2, then running this command: xkill