A Joke Assassin. Someone who interjects with a joke that completely kills the vibe of funny that is usually associated with a circle of friends that are joking around.
Can also be used when cracking jokes on someone who in turn takes it too literal and repeatedly ensures jokers that he wouldn't be or do what the Joking was about.
George: Gerald is always looking at zombie stuff.
Mike: I know, he'd totally make love with a zombie if he could.
George: I know, right! (Laughter)
Gerald: I would totally not make out with a zombie, she would bite me and turn me.
Mike: There you go again, Jassassin.