GNU/Linux is a stable operating system but some applications are not!
It could happen that an application doesn't respond to commands and it can't be closed using common method (a click on the close button or Alt+F4 key combination).
If you experience similar situation, you can close problematic application by running the following command in terminal:
sudo killall %application-name%
Note: If you can't even start terminal use Ctrl+Alt+F2 to F6 to display a console from which you could login and then eventually kill frozen program. After you kill it, you can return to the GUI by pressing Alt+F7.
In case that this command doesn't work use -9 flag to kill the application:
sudo killall -9 %application-name%