What is a word for an officious person who tells the ending of movie and mystery story, and spoils your interest?

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?

I think you would just call that person a spoiler.

spoiler |ˈspoilər|
noun
1 a person or thing that spoils something.
• (esp. in a political context) a person who obstructs or prevents an opponent's success while having no chance of winning a contest themselves.
• a description of an important plot development in a television show, movie, etc., before it is shown to the public.
• a news story published to divert attention from and reduce the impact of a similar item published elsewhere.
[NOAD]

The term refers both to the act of spoiling and the person who does the spoiling.

Besides previous answers, consider

• kibitzer, "A person who offers unsolicited views, advice, or criticism"
• meddler, "One who meddles or interferes in something not of their concern", a busybody

As @Robusto says, there is spoiler, which superficially looks like it should fit the bill. But I must I admit if someone started talking loudly about the plot twists of the film we're all queuing for at the cinema, I wouldn't be expecting to hear cries of "Be quiet, you spoiler!"

In practice, more likely epithets would be blabbermouth, bigmouth, blabber, blabberer, chatterbox, loudmouth, motormouth, squealer, telltale, tattletale.

My personal choice would be ratfink - rat and fink can both be used of a person who betrays secrets, and are both pejorative - the more so when used in combination, I feel.