Hi, I'm Ron Paul from HG, I'm not an actual member, but I do participate with their clan in the Jailbreak server, and I'd like to say that DDoS (Distributed Denial of Service) is a criminal offence, and is taken seriously. You can try to contact his Internet Provider, they'll probably drop his connection/report him to the authorities.
Please, don't hesitate to do this, it only enables him to do this to other communitys/servers. This is a serious offence, and he needs to be taught a lesson (Hard way. obv)