It's thought termination rhetoric designed to disrupt the flow of conversation. It uses consensus as its fuel. Ask them why that person is a figure of authority. When they explain why, ask them why someone else couldn't be an authority on the subject. Then dismantle the "degree" rhetoric, where groups of people approve someone being able to say they know things. Then you can work in to dismantling the academia in general and point out that science is still full of questions and that conse sus often holds us back and, in fact, we often make the biggest strides when it comes to deviance. Of course, you don't want to be a miserable twat no one wants to be around because you always start arguments, but this is for the sake of people thinking critically again, so it's important to choose these battles wisely.