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