Xkill is a utility for forcing the X server to close connections to clients.
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.
Source:xkill Linux man page