Report the player using the in-game reporting feature:
Highlight the player you wish to report while in the Game Lobby.
Press A (Xbox 360) or X (PlayStation 3) to bring up the Player Option menu.
Select Report Player.
Select the offense you wish to report from one of the four categories: Offensive Language, Exploiting, Cheating, Boosting
To report a player online, contact @IWEnforcers
on Twitter or email firstname.lastname@example.org