How to force a crashed application to quit

Method One:

Type in a terminal.....

top

...and make a note of the PID number of the process you wish to stop. Then type into a console.....

kill PID

...where PID is replaced by the actual number of the process.

Method Two:

Press alt+PrintScreen and type reisub to shut down and restart your machine properly.

Method Three:

Use the "Force Quite" Gnome Applet.