Killjoy

Killjoy is a medal awarded and achieved in Halo 3 Multiplayer. To obtain this medal you must kill somebody who is on a Killing Spree or any other known spree (Killing Frenzy, Running Riot, Rampage, Untouchable or Invincible). It is represented by an octagon with a crossed-out smiley face in the middle. This medal is very random as you don't know which players are on killing sprees, unless of course, he has killed you or your team multiple times without dying. A good way to see this through is to look for a sniper who is sniping other people. Go to an inconspicuous location and snipe him.

Another medal can be achieved by getting a killjoy on one of the Mythic Maps.