Killjoy

Killjoy is a medal awarded in Halo 3 Multiplayer. To obtain this medal you must kill somebody who is on a Killing Spree or any other spree (Killing Frenzy, Running Riot, Rampage, Untouchable or Invincible). It is represented by an octagon with a crossed-out smiley face. 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.