I think this article on How to Disagree is a very helpful resource for keeping discussions on the internet productive:

http://www.paulgraham.com/disagree.html

From the article:

"Truly refuting something requires one to refute its central point, or at least one of them. And that means one has to commit explicitly to what the central point is. So a truly effective refutation would look like:

The author's main point seems to be x. As he says:
<quotation>
But this is wrong for the following reasons...

The quotation you point out as mistaken need not be the actual statement of the author's main point. It's enough to refute something it depends upon."