biochemm

The one trick I've learned (mostly through analyzing why I respect some professors despite being subject to their scathing criticism) is to be honest and confident and deal with the behavior like a boss, but to never judge or make assumptions about the student's personality, motivation, or abilities.
Instructors who get the best results:
Might tell the student that his/her behavior comes off as [insert negative thing here], but will never say that the student is A or B or that they think the student is A or B. Might say why they took points off and where the problems are in a certain paper or assignment, but will never generalize the performance on that single assignment to make negative assumptions about the student's performance overall or question the student's effort or abilities. Keep personal feelings out of it. Treat each situation as something separate - once a conversation is over, let it be over. Don't let your attitude cloud your other interactions with said student or with other students. If they don't bring up the topic of a past unpleasant interaction with you, you don't bring it up first. You smile and treat it as if nothing happened after the fact. Also, it's okay to clue in the instructor of record if it gets too absurd.
As a TA, one of the worst interactions I've had with a student involved that student being aggravated by a not-so-great grade on her essay and was trying to get points that I couldn't give her. She yelled at me during office hours, refusing to believe that her answers weren't correct and telling me that she didn't think I understood how much money and effort she put into her education, that I didn't care because I wasn't paying nearly as much as her, and that I wasn't qualified to teach and probably didn't even read the textbook.
I ignored any attacks on me. I told her first of all that I didn't question her ability or effort - there were specific things we were looking for and she didn't have it; it didn't mean she was a bad student or that I graded on a whim. I also explained the answers to the questions and the reasons her answers didn't express them adequately. I then told her that she was welcome to go to the instructor for a second opinion (which she was going to do anyway). After she left, I followed up with an email recapping what I'd said, BCC'ing the instructor, and I met with the instructor later that day to talk more about the incident. When the student sent in the essay for that second opinion, the instructor and I went through it together, and the fact that the instructor was on my side about the grading helped the student come to terms with her grade, and she ended up apologizing for her attitude.
Honestly, I have to really thank this student. She gave me a killer answer to a really common and hard-to-answer interview question!