Debating a True-Believer™

Recently I was having a discussion on a Facebook group forum. It was an anti-vaccination group and I'd only gone there to look at some of the things they were saying, and what I saw amazed me. I had never seen such convoluted reasoning before in my life. People were giving anecdotes for why their particular belief was true, while simultaneously saying that they weren't basing their views off anecdotes. When pressed to give data supporting their position, they would go on a rant about how scientists are being paid off by big-pharmaceutical companies. When I would mention the fact that most medical scientists could earn a lot more money if they converted to alt-medicine, because people like them would buy any books they would write, they would have another anecdote about a scientist they know. When the nature of their argument was shown quite clearly to be circular, they would continue repeating the same things ad nauseum. Perhaps attempting to engage 'True Believers' is pointless......
What motivates me is that some of them are out there doing real harm to people, and perhaps if one person turns their back on medical quackery, then there might be one child who doesn't have to die in the hands of an incompetent homeopath