Xkill is a command to kill running windows and applications in ubuntu.Compare to kill,xkill is more convenient that you just need to select the window whose client you wish to kill with left-click.
You can create a keyboard shortcut or shortcut icon for launching xkill.When you press the keyboard shortcut or the shortcut icon,the cursor becomes a cross.Then just left-click on the window you want to kill
1.Create a keyboard shortcut:Go to SYstem->Preferences->Keyboard Shortcuts.Click Add,type xkill in Command area.Click Apply,then set its shortcut.

2.Create shortcut icon: Right click on panel,and select “Add to panel”.Double click “Custom Application Launcher” and type xkill in Command area.