Flag Carrier Kill
From Halopedia, the Halo wiki
Flag Carrier Kill is a medal awarded in Halo 2, Halo 3, Halo: Reach and Halo 4 multiplayer for killing an opponent who is carrying a flag. In Halo: Reach, it is also awarded for killing the carrier of the navigation core or power module in Invasion. Players who earn the medal are presented with the message: You stopped (player).
In Halo 4 it is represented by a white flag with a blue 'X' on a blue circle and grey four-pointed star. In prior games it is represented by a targeted flag on a gold-rimmed white octagon.