may be to say a witty and clever remark at the end,such as person1:so why was the communist kicked out of the restaurant? person2:why? person1: Because he was Stalin the line crowd:{laughing} person2h yeah,like the Cold War crowd:{glare}
there are many clever ways to kill a joke, but the best way is to always add something lame after the jokle or to just cut him off repeatedly while he's making the joke.
I'd say that the best way to kill a joke is... AIDs. That's still a very sensitive/'no go' topic for many and introducing it to a joke usually kills it very quickly.
If the joke is a pun, and it was a good pun, I tend to ruin the pun by saying "That was punny!" That'll get you some exasperated looks.
Then, of course, there's "I don't get it." It works especially well if you compliment it with a dumb look and a special tone.
Another way that comes to mind is to laugh obnoxiously or excessively. John: Hey Bill, what did the fish say when it hit a brick wall? Bill: I don't know. John: Dam! Bill: GAHAHAHAHAHA-- John: Get it, 'cause-- Bill: --HAHAHAHAHAHAHAHAHAHAAAAAAA HEEEHEEE HOOOO HAAAAA GAHAHAHAAAAAAAAAAAAA-- John: Bill, would ya-- Bill: --OOHH, OOHH 'DAM' GAHAHAHAAAAAA