A person or group of people, who take an activity so seriously and or literally, that they squeeze every last drop of joy out of it.
Can be appropriately applied to cyclists who spend 10K+ on their bikes, clearly don't enjoy what they're doing and live for looking down on those with 'lesser' equipment.
"Did you see those Joe Serious bellends shaking their heads and tutting?"
A person or group of persons who have the uncanny ability to absorb all of the fun out of something, not unlike a sponge...
Person 1: I say its such a wonderful day, shall we partake in a light spot of fornicating the establishment...
Mikeey: No..... *Sound of fun absorbtion fills the air*
Person 1: You sir are such a FUN SPONGE...