第一个是 a wet blanket.它的意思是"泼冷水的人,扫兴的人" a person who is not enthusiatic about anything and who stops other people from enjoying themselves

she was such a wet blanket at the party that they never invited her again.
她在那次聚会扫了大家的兴,之后再也没人邀请她了.

第二个短语是:a fly in the ointment,意思是"扫兴的人,煞风景的事物". a person or thing that stops a situation, activity, plan from being as good or successful as it could be

We lead a very happy life here. The only fly in the ointment is that there's too much traffic on our road.
我们在这儿过得很快活,唯一美中不足的就是住处旁边马路上的车辆太多了.