L.A.
I completely understand how you feel.
My father did this to my mom.. and My sisters ex did it to her..
If you want him to stop bringing her around your son.. remind him of the divorce decree and tell him you will take him to court if he continues to break the agreement. And then Do it if he does not honor your request.
If you do not want to put your energy or money into this, then.. I suggest you put on your best.. "strong woman in charge" face on and act as though butter could not melt faster in your mouth.. You will be so warm and kind, it will scare the poo out of her.. She will be looking behind her back for the knife..
When you see her and her son.. introduce yourself first, with "Hello Sally, good to see you again. What is your sons name? Hello______ I am A. (or I am Ms. L). I am (insert your sons name here) mom.."
And each time you see her, be gracious, try to always be the one that greets her first.. I promise it will freak them out..
You will be known as "Lady Gracious" and she will forever be known as the other woman who broke up the marriage.
People will wonder why on earth your ex left you, you are so amazing and kind..
The best revenge is to live well and be happy,,
FYI, this will also be a great gift for your son.. He will always remember that you handled the whole situation with grace.. And do not worry, he will figure out on his own what a skunk his father is.. Just sit back and wait.