In 12.04 How do you kill the X server from the command line and from the GUI interface and how do you start it from the virtual terminal?
-a means it will kill all display instances. For more information run
startx but this command is to start an instance and not the server itself. For more information run
man xint and
I don't believe it's possible to kill Xserver from GUI, and even if it were, it would not be the right way to do it.