Help! How do I deal with ex-husband's abusive second wife?

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Very long winded background history but my ex husband and I divorced about 4 yrs ago and he remarried. I was the one who ended the marriage and felt nothing but relief when it was finally over. During the whole time my ex husband was with his second wife she was abusive towards me, physically and verbally, threats to kill and so on. It went to court regarding my ex's and my son (who is now 6) about contact oders and was shown that my son was being neglected when in the care of my ex's new wife. Anyway, my ex had a daughter with his second wife 2 yrs ago but they broke up and he now lives elsewhere for the last year. Last night I had a horrendous phone call from his second wife, who called me every name under the sun and some and when she finally got to the point of her call, it turns out my ex has told her he can only pay a lesser amount of maintenance (child support) for their daughter as the CSA has forced my ex to pay a large amount weekly for our son. He's not paid a penny in 3 yrs by the way and this will be the first money I get from him. I cannot reason with this woman and I spoke to my ex straight after the phone call. He says he's resigned to paying for our son and that he's not interested in getting involved with arguments between us women. So now I don't know the best course of action to take. I'm angry with myself for getting very upset over it all and thought it was firmly in the past when I wouldn't have to have any more contact with her. Any suggestions on how I can put a stop to her abuse? I can't face court action again