Showstopper
From Halopedia, the Halo wiki
Showstopper is a Halo: Reach and Halo 4 multiplayer medal awarded to a player who kills an opponent while they are performing an Assassination flourish. The medal is represented by an an octogon with an image of a combat knife crossed out in a "No" symbol. It was first revealed alongside two other medals in the Bungie Weekly update 07/09/10.[1]