Gone to a supervisor and said, "While I was gaming over the weekend, I noticed a user at this IP address engaging in illegal denial of service attacks direct at multiple users on XBox Live!, including me. I think this is a Comcast IP address. Can we investigate this and move this up for action by Comcast if it is a Comcast user?"
If he had a decent relationship at all with his supervisor, the matter could have been handled professionally and still resulted in cutting of the internet access of the original loser.
I think that conversation would have some something like...
said by Supervisor :STFU douche. Gimmie another Twinkie 'n' get back to work. And for Christ's sake put yer friggin' hat on straight. You look like a 'tard.