Party pooper
A party pooper was someone who tended to put a damper on fun, such as at a party.

In 2375, Seven of Nine chided The Doctor for bringing a holo-imaging device on an away mission to take a few snapshots, noting that they were not on shore leave and that there would be no time for recreational activities. The Doctor retorted by citing the Borg as "party poopers of the galaxy." ()

See also 
 Cold fish
 Stick in the mud

Societal classifications
Insults