'Sobering' is a less colorful and more polite word to capture the effect.

“Buzzkill” isn’t bad or rude at all. It is very casual, though.

A "wet blanket." For example, "I don't want to invite him to the party because he is always such a wet blanket." (He is not fun, and he ruins the good mood.) Good luck!

A newer expression meaning the same is "Debbie Downer". You can find old Saturday Night Live sketches with their Debbie Downer character to illustrate this concept.