What can be done about it? If a quack consistently fails to cure people, it's only a matter of time before those people (or their survivors) get angry and seek revenge. Not that I condone murder, but it's not entirely unexpected. To avoid being murdered, restrict your nonsense beliefs to a domain where people won't get sick or die from them.
I don't believe that is true. After all, the quacks are the standard medical. They kill hundreds of thousands people every year by accidents they make in diagnosing, prescribing and administering drugs. They kill many more by allowing deadly drugs without controlling their patients personal use of the drugs. Yet nobody goes around killing off medical people. In the same way they wouldn't be killing off holistic doctors who actually cure diseases.