Another option you can employ is xkill. This is preinstalled in Ubuntu, but you can install it via the Terminal (which you can open with CTRL + ALT + T) if necessary. When called, xkill will enable you to close any desktop app. Install it with the following command:
sudo apt-get install xorg-xkillOnce this is done, run xkill by simply typing
xkillYour mouse pointer will then display a cross (or a skull), and all you need to do is left-click on the offending application.
If it’s not possible to close your unresponsive app with any of these desktop-focused methods, the solution might be the command line…
soultidis blog 