If you don't like command line, you can kill an application with xkill. Press ALT+F2, type xkill and hit the Run button. The program will displays a special cursor (usually a crosshair or a skull and crossbones) and will display a message such as
Code:
Select the window whose client you wish to kill with button
If a non-root window is then selected, the server will close its connection to the client that created that window, and the window will be destroyed.
Please note that xkill is not intended to be used as a routine way to terminate X client programs, but only as a last resort to abort malfunctioning or malicious clients.
Unlike kill, xkill does not request that the client process, which may be running on a different machine, be terminated. In fact, the process can continue running without an X connection. Most clients, however, do abort when their X connections are unexpectedly closed.