It means, that you have killed an enemy, who was 1 kill off from killstreak. (example if enemy has 4 kills but needs 5 kills for predator missile and you kill him, so he wont get the 5 kill reward.)
It means that you kill a person and stop them from having a killstreak
A buzzkill is something or someone who spoils an otherwise enjoyable event.
To upload it to mw3
Buzzkill - 1996 1-1 was released on: USA: 13 May 1996
Buzzkill - 1996 1-2 was released on: USA: 18 June 1996
Buzzkill - 1996 1-3 was released on: USA: 25 June 1996
Buzzkill - 1996 1-5 was released on: USA: 9 July 1996
Buzzkill - 1996 1-6 was released on: USA: 16 July 1996
Buzzkill - 1996 2-1 was released on: USA: 17 September 1996
Buzzkill - 1996 2-2 was released on: USA: 24 September 1996
Buzzkill - 1996 2-3 was released on: USA: 1 October 1996
Buzzkill - 1996 2-4 was released on: USA: 8 October 1996
Buzzkill - 1996 2-5 was released on: USA: 15 October 1996