How could I make sure she will keep her distance from my son?

I got my divorce 3 months ago , because he cheated on me, his mistress was his co worker or I should say secretary for many men,, before became my husband mistress she was the mistress of another  man in the same group, when he left, she became the mistress of my husband,  they were doing this behind my back and just laughing at me well after one year of battle I got my divorce, but I am suffering  a lot, I do not know why I am suffering ,I did not do anything wrong I was a kind and good wife  and mother I gave more than my 100% to my family, but now I am crying  a lot and specially  when I discovered that they have not  been together for 2 weeks and she e mail him and telling him that how he can even think about going back to his family and so on at the end she says this time he is saved but...., then he answers back and says many times sorry.  I really do not want this woman around our son. she  does not have any conscious and moral. she is very selfish she is only thinking about her own pleasure well at the age of 58 I think my husband was her last chance, and do not know exactly what she has from him but whatever it is has made him be depend on her.  I do not have her experience about men at all, my husband was the only man in my life, and I think may be that is why I lost him, I lost also the father of my child which hurts even more, I am so alone at this thing and I blame myself a lot to put my son in fatherless situation.  I have full custody of our son but he can come and get our son every other week end, for now he is living alone but I am afraid if they are together my son will be expose to that kind of immorality .Legally what I can do to keep her far from our son, please help me, I need to know what kind of document I need to present at court to get restrain order against this woman.