Not exactly a killjoy or a wet blanket or a Republican. Those may be people who don't want to see you having fun, especially at their expense or inconvenience. Republicans do know how to have fun. Some of them go to the theater, some go hunting, some fly planes, some visit their Swiss bank.
A sourpuss or grump is someone who doesn't know how to have fun.
Stick in the mud; grump; Scrooge
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