I've decided to treat these claims just like any other claim in science. If someone claims that the world is 6000 years old, I'm going to ask for scientific evidence of this claim. If they decide to provide some, then I'm going to compare it with what exists that shows the Earth is much older. This is going to debunk the opinion, but I think that it's also going to teach some important lessons.
1. When is a person wrong and when is a person right? Often times in our society, in order to be inclusive, we try to embrace every position equally. This doesn't happen in society. Science is a competition of ideas. You are not entitled to believe something just because it's important to you. If the reasoning doesn't stand, then according to the rules, it must be discarded.
2. That this process can be done without ridicule. Rarely does the media show a calm, rationale, and respectful person winning an argument. Mostly, all we see is the cocky, brash, and haughty person taking care of business in such a way that his opponents slink away embarressed. Modeling the difference between former and "pwnage" and showing them that this is how adults should be arguing with each other is a good lesson.
One of the things that I strive to do in my classroom is to create an environment where people are welcome to share their opinions. I try to make it a safe place, free from ridicule, so that everyone it welcome to speak. Hopefully, the comparison between viewpoints won't jeopardize this.