The most common and most likely answer to this crossword clue is the 8 letter word KILLJOYS
I've seen this crossword clue in The Guardian
K
I
L
L
J
O
Y
S
Buddy explains!
The straight part
We know that "such guys spoil things" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case such guys spoil things = killjoys.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does KILLJOYS mean?
KILLJOYS noun- A person who is anti-fun, or prevents others from having fun.