Party Pooper
A person whose behaviour or personality spoils other people's enjoyment
Party pooping tends to involve someone who loves to come into conversations where people are being crazy and having fun and telling them that they're being ridiculous and stupid. This behavior often occurs in adults who didn't have fun when they were kids or young people who want to desperately be adults.
Examples of party pooping posts:
(Note that, these posts were only made to tell people to stop having their zany fun.)
Wow, you guys don't know how to not stuff post. Take your roleplay stuff into a chat room, seriously.

And once again, the stuff posting continues.

Seriously, you need to stop spamming up topics with your stupid stuff and let people have their fun, I'm tired and I'm sure others are, of you doing this kind of thing.