xkill [-display displayname] [-id resource] [-button number] [-frame] [-all] [-version]
Xkill is a utility for forcing the X server to close connections to clients. This program is very dangerous, but is useful for aborting programs that have displayed undesired windows on a user's screen. If no resource identifier is given with -id, xkill will display a special cursor as a prompt for the user to select a window to be killed. If a pointer button is pressed over a non-root window, the server will close its connection to the client that created the window.
This command does not provide any warranty that the application whose connection to the X server is closed will abort nicely, or even abort at all. All this command does is to close the connection to the X server. Many existing applications do indeed abort when their connection to the X server is closed, but some can choose to continue.
X(7), xwininfo(1), XKillClient(3), XGetPointerMapping(3), KillClient in the X Protocol Specification
Jim Fulton, MIT X Consortium Dana Chee, Bellcore