using just rm -rf ...
But my concern is more I never saw a message that is untrue when it comes to a directory, so what is it? If its a mounting point and you remved lost+found, you are looking for trouble... But if it is try to umount it, it will tell you if it can, in your case it should not be able... which means and explains the not empty, someone removed a file that was still open... so you are to use fuser -cku to kill whatever process his there...