Your comment suggests an interesting way of rewarding people who further the conversation. Give the user their upmod points of the entire thread that responds.

1 - This makes the witty comeback a lesser strategy for obtaining karma.

2 - People who are aware of this will stop responding to trolls. Even if they publish a well-reasoned response to a troll, they will be giving them karma points. Since this is not a common feature of internet forums, this might be worth spelling out above an active comment form.

3 - The best strategy for karma is to advance the discussion somehow. Even if your point ends up exposing the point of view of people on the other side.