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They say not to feed the trolls, and sometimes you just have to let it go.
But there have been a few cases where my response appeared to be effective.
For instance, there are grammar/spelling nazi trolls. My response is something along the lines of, "The guy made a common mistake, this forum is not a peer-reviewed journal where you have to be particularly careful, and the rest of us were smart figure out exactly what he meant. Are you too stupid to work it out for yourself?"
Then there are the party poopers, and I respond with something about them being responsible for taking the fun out of having discussions in comments on forums. "It's people like you who make the internet suck. Why do you have to make it your role in life to suck the fun out of everything for everyone else?"
In none of the cases where I have said such things have I gotten a response. However, It's possible that I was being trollish a bit myself. I'm not sure. I don't do this often, but a number of times, someone has just stuck out to me as being especially jerky and needed to be put in their place. No idea if it really had any positive effect.