Xkill

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.