Please use the !votekick and !voteban commands to get rid of griefers.
Remember however that the way the Halloween event is designed, Valve obviously intended for teams to be allowed to kill each other.
Server operators were punished last year for "farming" the Halloween boss and cheating the gift system. When an admin is available, she/he can help making sure players are not actively griefing each other, but it will also be up to players to work together to kill the boss.
You can also type !request in-game to ask the help of an admin. We're not available 24/7, but we try