User blog comment:Dick-Winchester/Closing the gates of Hell will keep demons away?/@comment-1509826-20130429203917/@comment-3481504-20130429211018

Slaughter? No, there's no need. Yes, the Levis could slaughter any demon, but they wouldn't have to because the door would be shut.

Kevin specifies, by saying, "Slam the gates of Hell forever." This means every gate to Hell will be closed forever.