"You killed it" is what one says to the other who puts an end to a funny joke.
People will be laughing at a certain joke, then someone will try to enhance the humour by adding a witty comment or something(see list below) to get more laughs albeit failing miserably and making the joke not funny anymore.
The most common ways to kill a joke are to tell the joke so often to the point it isn't funny anymore, or to attempt at enhancing the humour by displaying a sarcastic/witty comment, taking the joke literally or stating the obvious.
1. "Why'd the chicken cross the road?"
"Dude, cut it out! You killed it a long time ago, you told the joke so many times!"
2. "why'd the chicken cross the road?"
"to get to the other side"
"maybe the chicken needed to go to the bank!"
"(sigh), you killed it."
3. "What's the difference between a pizza and a Jew?"
"A pizza doesn't scream when you put it in an oven!!!"
"Well why would anyone wanna put a Jew in an oven? They're humans too!"
"It's a joke dumbass, you killed it..."
4. "How do you get 4 fags to fit on one bar stool?"
"Turn it upside down."
"HAHAHAHHAHA!!!!! That's a good one eh?"
"Wow, sounds like they must've enjoyed it, huh?"
"You killed it again!"