Come up with an absolutely ridiculous punchline before the person telling the joke tells you the actual punchline. Since your answer will most likely be funnier than the real answer, they'll feel stupid knowing that you're laughing at them on the inside.
Actually, I think it would be worse if you explained the joke to the joke teller, especially if you explained it incorrectly.
The best way to kill a joke is to punch the joke teller in the face before they can finish their joke, and then say, "HA! Beat you to the *punch*line!"
It not only kills the joke, but it makes the person impatiant to hear the end so they can get on with there life where you are not laughing like a bafoon.
P1: Wanna hear a joke? P2: Kay. P1: Knock knock P2: Come in! P1: *Smacks forehead* That's how you ruin a joke no. 1
P1 and P2 then tell a successful joke. P3: *Walks in* P1 and P2: 'Sup! P3: Overheard your guys' joke. It'd go much better like this... Correcting jokes isn't cool. It's really freakin' annoying.
Okay, a little off topic, but because I'm the OP, I can do whatever I want. But it still relates to jokes.
Q "Why did the monkey fall out of the tree?" A ""Cause it was dead." Q "Why did the second monkey fall out of the tree?" A "Cause it was dead." Q "Why did the third monkey fall out of the tree?" A "Peer Pressure."
Okay, a little off topic, but because I'm the OP, I can do whatever I want. But it still relates to jokes.
Q "Why did the monkey fall out of the tree?" A ""Cause it was dead." Q "Why did the second monkey fall out of the tree?" A "Cause it was dead." Q "Why did the third monkey fall out of the tree?" A "Peer Pressure."
Being the creator doesn't give special privileges in that regard. At least it's not way off-topic.