Instructions
Press the "Guide" button on the Xbox controller to view the Xbox Guide.
Select "Friends" on the Xbox Guide.
Press right on the directional pad to highlight the "Players" tab. This tab lists the players you have recently played with in a game. Scroll down and select the player you want to report. The player's profile appears, along with a list of options.
Select "File Complaint." Select "File Complaint" again to confirm your choice.
Select "Profile" from the list of reasons for filing a complaint.
Select "Gamertag" to report an offensive gamertag.
Select "File Complaint" to submit the complaint.