How can I report someone who stole my identity?

Hi, folks! I'm in a desperate situation. I want to report as spam a girl who stole my identity. She made a yahoo id, she uses pictures with me as avatar.I intend to claim her at the courthouse, but at the same time I want to stop her to pretend that she is me, so I need to know how can I block her yahoo id. If someone can tell me, I will be very thankful!