wet blanket

a wet blanket (a person who'd spoil general pleasure)

зануда, кайфолом, кайфоломщик

a spoilsport

Before that spoil-sport Hetty came in and so rudely interrupted us. — Прежде чем эта вечно портящая удовольствие Хетти вошла и грубо прервала нас.

[party pooper]

a killjoy
a buzz-kill, a buzzkill

(a person or thing that has a depressing or dispiriting effect or takes the pleasure out of an otherwise enjoyable experience; a killjoy) — кто-то или что-то, что разрушает веселье; тот, кто вечно портит настроение, обламывает; кайфолом

... He is a real buzz-kill.
... I thought the first hour was fun, but the second hour was a definite buzz kill.

crape-hanger (Am)

stick-in-the-mud (one who does not go out) - скучный занудный домосед (sometimes also called "ботаник" (nerd), though "ботаник" refers to a 'high-brow smartass' mostly.

party breaker (a person who leaves first, in the middle of a party).

Once one person broke the party made in her honour.
know that some people hate such parties, but its better not to come to a party than to break it intentiously.

Related vocabulary:
[party pooper]
[rain on sb's parade]
[burst sb's bubble]
[портить удовольствие]
[обломать кайф]