Since many conspiracy theories arise from feelings of uncertainty and fear, an angry debate will only cement the ideas, and open ridicule is even less constructive (see panel, below). Instead, the research shows that you should try to focus on the rhetorical devices and tricks of persuasion that have been used to spread the ideas in the first instance.
It would not be constructive to go into the conversation in a hostile manner, because this delegitimises their concerns and might alienate them even more.