"You are not obligated to answer the question. Rather, use the question as an opportunity to advance your ideas." This is commonly known as the Politician's Rule #1, but it works for speakers as well. It works for the politicians to help them gain power, but it works for the speakers for a more noble reason: Your prime responsibility is to your audience, not to any single audience member. And your audience each gave up an hour or more of their time to come and hear you speak, so it's pretty certain they're more likely to be interested in what you have to say than what the random audience member has to say. So you're showing respect for your audience by keeping the topic on track.
An excellent example of turning the discussion comes from Jesus: His adversaries asked "Is it lawful to give tribute unto Caesar, or not?" But Jesus responded "Show me the tribute money." When they brought him a penny, he asked, "Whose is this image and superscription?" They responded, "Caesar's." His response was "Render therefore unto Caesar the things which are Caesar's; and unto God the things that are God's."