Husband's exwife likes to call me names on facebook. What can I do about it?

My husbands ex wife is a bully. She has never liked me from the start. I helped raise her son and put up with her bulling for 8 yrs. Now she doing it on facebook on her son's page that has pictures of him. Everytime I make a simple comment, she writes me back with harassment. I saved one of her emails because it was so hurtful and packed with insults towards me. Two months ago, she sent me a friend request wanting to be my friend, which now she denies that. *****. Anyway, what can I do about all of this? I love my stepson and we are friends and I want to be in his life now. Unfortunately, his mother and I are linked together on his page. I have to be the bigger person there and watch what I say to him in fear of her insulting me. I need help and someone to assist me in fighting her with some smart criticisms. I want to put her in her place for good. I have never bullied anyone. I was not raised that way and this has got to stop.