My DD's questioning is a mixed bag, and it's not always obvious why she's doing it at first. So, our default position is to explain. Once the conversation starts going in a circle, that's when it becomes apparent that she's just questioning because she doesn't want to do something, and she's being oppositional. That's when I attempt to end the conversation, with signal phrases like, "I've made my decision," and "This conversation is over," which lets her know from prior experience that to continue to question me will result in privilege losses.
Of course, whenever she's questioning over well-traveled ground, we go right to the aforementioned, "Why are you asking questions when you already know the answers?" and "You already know why." Then I signal my disengagement by becoming monosyllabic: "Stop." "Go." "Now."
As for dealing with other authorities, DW and I have made it abundantly clear to her that when she's under someone else's supervision, we will only intervene in significant circumstances. We will not punish her for misbehavior under their care as long as it's something they can already punish her suitably for, and we will not advocate on her behalf unless it's an important issue, and then only after she has already advocated on her own. Within those boundaries, it's her relationship with them, and she'd better figure it out.
