If someone takes Polyjuice potion, can the effects be reversed before the one-hour potion acting time expires?
For example, by applying a specific counter spell, or stunning a person? Or killing them, if you're a bad kind of wizard?
I don't believe there is an example of a counter-spell, but at Gringott's there is the "Thief's Downfall". On page 534, of HP and Deathly Hallows, Scholastic edition:
...to Harry's horror he saw that she [Hermione] was no longer Bellatrix
As Griphook explains:
It washes away all enchantment, all magical concealment!
So while there isn't an example of a spell being used directly, the Thief's Downfall can counteract Polyjuice Potion.
I don't recall any mention of a spell designed to reverse the effects of the potion in the books or JKR interviews (though someone needs to check Pottermore Chapter 12 Moment 2 to make sure nothing is mentioned there).
As a very strong evidence we can see that in DH, even the most skilled and senior members of the Order of Phoenix are forced to use a decidedly muggle challenge-response approach to detecting possibly-Polyjuiced-doubles after the Battle of Seven Potters (or when Lupin visits Bill's cottage).
However, it is 100% certain that neither stunning a person nor killing them (to use your two examples) will affect the transformation.
Barty Crouch Jr. was stunned by Dumbledore in GoF, when he was trying to kill Harry inside the room. He remained looking as Moody, until Dumbledore waited a bit.
"Stupefy!" There was a blinding flash of red light, and with a great splintering and crashing, the door of Moody's office was blasted apart. Moody was thrown backward onto the office floor.
"Polyjuice Potion, Harry," said Dumbledore. ... But I think, in the excitement of tonight, our fake Moody might have forgotten to take it as frequently as he should have done ... on the hour . . . every hour. . . . We shall see."
Minutes passed in silence... Then, before Harry's very eyes, the face of the man on the floor began to change.
Barty was busted out of Azkaban by virtue of his mother taking the potion to look like him and stayed in prison till she died (and died looking like him).
"My mother died a short while afterward in Azkaban. She was careful to drink Polyjuice Potion until the end. She was buried under my name and bearing my appearance. Everyone believed her to be me."
At the end of Goblet of Fire when Barty Crouch Jr. was exposed, Dumbledore states that Crouch had been using Polyjuice Potion:
"Polyjuice Potion, Harry," said Dumbledore.
Shortly thereafter, he states that Crouch may have forgotten to take it frequently enough, and he waits for it to wear off:
But I think, in the excitement of tonight, our fake Moody might have forgotten to take it as frequently as he should have done... on the hour... every hour.... We shall see."
Dumbledore pulled out the chair at the desk and sat down upon it, his eyes fixed upon the unconscious Moody on the floor. Harry stared at him too. Minutes passed in silence....
Then the potion actually wears off:
Then, before Harry's very eyes, the face of the man on the floor began to change.
If there was simply a way to cast a spell to reverse the effects of the potion, Dumbledore surely would have done so rather than wasting time waiting for it to wear off. Apparently, then, assuming Dumbledore is not a fool, (which may be a big assumption) there is no way to simply cast a spell to reverse the effects of the potion.