If a person dwells on a question, and you really don’t want to go there, simply say, “That’s a great question. Because I want to respect the time constraints that we have here today, what I will do is go ahead with the presentation and answer that question for you in the back of the room at the end of the session. For now, let’s continue with…” People will often try to stop you with difficult questions solely because they want to have airtime. They will rarely meet you in the back of the room afterwards.
Read more on how to handle difficult questions by looking at the article How to Handle the Disruptive Person That Asks Lots of Clarifying Questions.