Hckers

can someone help men Im very sure someone hcks on crossfire but i dont know what to do his name is [sp]ecialist and i saved the replay and when he shot a lot of the time he missed but the bullets still hit the people and he was headshotting everyone from across the map with an m4a1 which the way he was doing it wasn't really possible so can someone help me to report this guy cuz idon't know what to do.