Well, on "civilized" forums people usually quote the text they are replying to, or at least call their opponent by his/her nickname, in case their answer is not immediately following the message they are replying to. It's that simple! Just click "Quote" instead of "Reply", an delete the irrelevant or excessive quotation to concentrate on what you're going to say. So there is no anything like "threaded vs flat mode" discussions or arguing happens ever.

By the way, one needs just 2 mouse clicks to switch from/to threaded/flat mode: "Topic Options" menu near the top of this page.
(Just tried that... Oh no! I'm lost. I'd better keep it flat