A fun sponge refers to a joyless or grumpy person who soaks up all the fun out of an event or situation, in a way a normal sponge sucks up liquid around it.

Fun sponge指无趣又坏脾气的人，能够“吸干”周遭环境中的一切乐趣，就像海绵吸干周遭的水分一样。

这个说法的字面意思是“欢乐海绵”，但实际上是破坏欢乐氛围的海绵，所以，我们可以称之为“欢乐终结者”。

For example:

Why did you invite him to the party? He's such a fun sponge. He is complaining about everything at the party and arguing with almost everyone he meets.

你干吗请他来派对啊？他就是个“欢乐终结者”。他一直在不停抱怨，遇上谁就跟谁起争执。