As intoduce in this post, you can kill any process with its PID. But what if you want kill a GUI-based program? You can use this:
This message will be show up when ‘xkill’ is executed:
Select the window whose client you wish to kill with button 1....
and you will get a ‘skeleton’ mouse cursor. You just need to click on any GUI program and all the related process will be killed as well as GUI will close.