Myspace Slander & Child Custody

I am needing advice. My son lives with me mainly, but my ex husband has our son more days a week than I do. My ex remarried, but got my son more than me even while he was living with the woman and we were still married.
Now she is posting things about me on myspace that are untrue. That I use drugs, that I slander my ex to her when I don't, that I am a bad mother, she calls me filthy names, and she says that she joined my myspace and that I share things with her, which is also untrue. She even goes so far as stating that she wants to take my son from me all together, and she says things about my family and friends. She bashes my lineage etc.
I have screenshots of her blog posts.
What can I do?