In Linux -as in all modern Operating Systems-, applications become unresponsive and you may want to kill the process pertaining to them. To do this, open the terminal and write this:
killall applicationName
If that does not work try
killall -9 applicationName
If you don't have an idea about the application name, you can take look at the current processes by writing
ps
then to kill the processes you want
kill processID
If that does not work then try
kill -9 processID
Another possibility is to use xkill (for applications with gui). The window of the application waiting to be killed should be visible. Then, open Terminal and type
xkilland later choose the window of the application to be killed.