It's not just the nonsense-spouter you need to worry about though. If that person has written a book, someone may be a "believer" in the nonsense because they don't know what's wrong with it. Von Daniken made a lot of sense to me when I was in high school and knew nothing about ancient history or anthropology. Ridiculing people tends to turn them off, rather than making them receptive to what you are trying to tell them. The best debunking has to be fairly serious IMO.