Buzzkill

[Donald Trump]

Something that spoils or ruins an otherwise enjoyable event, esp. when in relation to ruining a drunken or drug-induced high.

[Sonia]

Buzzkill (n. or v.trans.)

Buzzkill, anything that takes the edge off your fluff, a downer, something that ruins your good mood; a killjoy.
(verb) to selectively and deliberately bring down someones mood by actions that are designed to irritate, annoy or downright upset a person.

[Dave Paprocki]

When you are high, drunk,or both and want to do something fun and some one you are with starts complaining about everything.

[Justin Bieber]

As the other definitions have said.

Can also be someone or something that spoils the mood for just about anything.