Xbox 360 - Stop someone talking to you in a party?

My son is 9 and he and his friends play minecraft and other games on the Xbox. They usually have a party and all play together. There's one boy who is their school friend but can get quite nasty. My son has un friended him but this other boy can still speak with him when they are playing in a party and is verbally abusive/bullying. Is there a way we can block him so my son can't hear him even though they aren't xbox "friends"?

You can go into the party and set it to "invite only" so no one else can join unless they get invited. I had the same problem.

But is that only if it's your party? What if my son is joining another friends party and this other boy is on there?

You can select mute on the mean bully on his profile page and you can't talk to him