Oxford Advanced English Learners’ Dictionary gives ‘wet blanket’ as a noun meaning;
A person who is 'not enthusiastic' about anything and who stops other people from enjoying themselves.
Is there a short word for a person who is 'too enthusiastic' to spare his or her fun in something (e.g. movie, detective story) with others and spoil their interest in it by revealing the ending of the story before others see the movie or read the novel?