Oooooh, what a little monster! The other ladies have good ideas and I was thinking the same thing. Put him in a playpen in the next room so he's completely removed from the other children. I wouldn't give him any toys though. Let him know it's definitely a punishment. Removing privileges for a favourite activity or removing a favourite toy from play are also great ideas. It has to be something that is important to him so he learns that what he is doing to hurt others means he suffers too.

Do you use a really stern voice and do the parents? The smaller the child the better the serious change in voice gets through to them in my experience. I also have the child go back and hug the person they hurt and apologize. It's good for them to look the crying child right in the eyes to see that they have caused someone else pain. Otherwise, best of luck to you.