You can enable grab break actions with "setxkbmap -option grab:break_actions" and then use Ctrl+Alt+KP/ to break grabs and Ctrl+Alt+KP* to kill the applications with grabs (note that setxkbmap -option blah adds the blah option to the existing options, which might interfere with this - you can try and reset all options by typing just "setxkbmap -option" without a parameter and then typing "setxkbmap -option grab:break_actions").

This is useful both when debugging and when dealing with broken applications.