What to do with disrespect-full child?

I have 4 children 3,4,10,and 12 year old all are very well mannered except mt 12 year old I get no respect from her what so ever. I use a rewards program, and do have an open relationship she can tell me anything which is where I feel I have run into a problem she does not sensor at all. To the point of being very disrespectfull. And I do not know how to over come her mouth, other than that she is a great kid. but her mouth!!! I have tried taking things away, she was in an empty room with nothing but a bed, tried grounding her, tried explaining to her what she is doing is hurting me, I think that I have tried everything short of beating her. I dont know how to make my child respect me.