Hilchos Rechilus, chapter 9, section 6
Even when there are two people, Shimon would sin if he acted upon his own initiative for
he may not believe them unless they testify in a beis din and the beis din gives him permission
to act as explained earlier in chapter 6 paragraphs 9 and 10. Also, it is rare to find such cases
(besides the fact that it is rare that all the conditions will be fulfilled) for it is very unusual for the
speakers to know all the relevant laws and know whether Shimon would be acting according to
Torah law. Therefore, even two people should not say anything to a person who generally takes
the law into his own hands without first obtaining permission from a beis din.