When the 9/11 truthers were spreading their bull and it finally managed to touch a few of my coworkers, I found my indignant attitude that anyone could believe something so obviously false was far better at convincing people of the truth than reasoned argument. Unfortunately, people are pack animals before they are rational beings. Probably the best way to respond to such claims as "vaccines cause autism" is to just say, "Wow, I didn't realize there was anybody left who still believed that." It's mean, but it's for their own good, and it works quickly. They don't get as upset as you think they would.