> Genuine questions: what can be done in a democratic setting to stop him?

Thiel is only "relevant" because he's wealthy.

In a system that allows wealth to equal political power, systematically weakening the impact of wealth on civic and political systems is an effective method. Whether that can be done in America, with the current understanding of the constitution and the current philosophy that many take towards taxation/wealth is questionable; but the idea that we can do nothing is just not true. We don't need to slide back into an era of 19th century robber barons and pseudo-aristocracy. If we do, it's because we largely gave up or allowed it to happen.