Yes it can. In the context of a law class, there's tons of laws which clearly state that rape is a crime, so it clearly does matter. That's just as factually incorrect as saying "Fraud does not matter because suckers aren't important." You can't just say: "_______ isn't important." in a law class.
If I were a history professor and someone tried to bring up Holocaust denial in my class, I'd first explain why it's absurd to try and argue that all that evidence is somehow fake, which then shows that anyone who tries to defend this position either:
1. Has legitimately no idea what they're talking about.
2. Has willfully taken up the agenda of bending the truth to support Nazi-ism.
I wouldn't shout them down though. That's counter-productive.
Oh, and now you want to argue meta-ethics and meta-philosophy :-p?
That isn't something that can be argued against with facts. You can't present evidence to counter this, because they didn't use facts to come to the opinion. As the saying goes, "You can't reason someone out of a position they didn't reason themselves into"
There are a certain class of opinions that we need to be able to dismiss with just a "that is just not worthy of even talking about." While I believe this list of things should be very small, it DOES exist.