T.F.
I am not an advocate of spanking anyone at anytime. That is just my personal feeling about hitting a child as punishment.
That said, your boy lives the majority of his life with dad and stepmom for whatever reason and she is the primary female parent in the household. Your ex and she established their own household rules and that is their business. You feel as though she is entitled because you are angry and bitter.
Your son got a good punishment and the fact that she had him write a letter is another good step. What your kid did was wrong and I likely would not have stopped at an apology and loss of electronics if it were my kid who did something so cruel.
First... calm down. Look at the big picture here. WHY dis your son do this?
Second... communicate with your ex and try to work with him because he is trying to raise your son to be a good man and your son royally f'ed up. Your son knows better than to do what he did and he may need some sort of counseling to find out why he feels the need to hurt others.
Third.. appreciate the fact that your son's stepmom cares. He could easily have a stepmom who didn't care about him. Your child has mom, dad and stepmom who care about him and want him to grow up to be a responsible adult.
You do not "need her to learn" anything. You NEED to work together with her and your ex to give your son the support he obviously needs. The fact he is a bully, especially to a special needs child, is a big red flag that he needs you ALL to help him.