Open a Terminal, type top, get the PID number of the offending app, open a new terminal window type kill -9 and the PID number.
I've seen some apps survive that too, not much you can do but try and log out. Sometimes you can't even do that.
You could kill loginwindow, that'll kill everything and drop you back at the login screen if I'm not mistaken.
Not a good thing to do at all but if you must...