buzzkill

noun

a) Something or someone that spoils an otherwise enjoyable event.<! verbose; extraneous material or material that belongs elsewhere: Initially, the term referred to a rapid bring down from an alcohol or drug-induced buzz. However, the term has moved into broader culture. For example, "The meeting was going great until our boss got up on stage and let the air out. What a buzzkill!"

b) A person who attends a social event and occupies his or her time with other activities than those offered at the event.<! verbose; extraneous material or material that belongs elsewhere: Someone who attends social events and dedicates time to events other than those presented for the group to enjoy. Examples include: doing homework, chatting with significant others over the telephone and Playing single player games with headphones.

Syn: dampener, killjoy, party-pooper, bother, bug, hassle, irritate, bring down, harshing my mellow