I too, am at my wits end! My 10 year old daughter won't stop lying. She lies about big and small issues. She lies all the time. She would be considered a pathological liar. She will look you straight in the eye and tell you she didn't do something, then she will start crying. When we pull the video, she of course, is caught doing it. We show her the video, and she still insists, she didn't do it. When she finally breaks down and admits to it, she starts yelling and then stomps away!
She has been punished for lying more times than I can count. She has been grounded, grounded to her room, grounded to her bed, lost all of her toys, lost her "treat", spanked, given more chores and countless other things. Nothing seems to work.