This may be unpopular, but this sets of my "Dark Arts" detector something fierce.

It is pure Dark Arts... but that doesn't necessary mean it is a bad thing. Just that is normal social behavior.

For my part I do tend to notice this move and cooperation mode gets shut down far more completely than if they simply disagree. But that doesn't mean I'll come out and tell them that I've stopped cooperation or even act less cooperative. The mode being shut down is 'cooperation with an intellectual peer'. They have taken the role of persuader with some sort of social agenda. There are all sorts of ways to handle that sort of situation and relatively few involve giving them free access to any more honest expressions of your own beliefs. Pretending to go along with them and so giving them no target to 'persuade' against is probably a better default.

It's always seemed to me that the respect I owe to my opponent in a debate obligates me to at least say, when I think it's the case, "You're wrong. You're an idiot. Think again."

I like the way you have framed that. You describe direct blunt disagreement as something you are giving the opponent out of (a certain kind of) respect. This allows for far more freedom when dealing with people who (at that particular instant) do not warrant that kind of respect.