Mormon explains that while the people might have forsaken Nephite religious ideals, they did still hold to the concept of the law. Rather than mob action, they reported the prophets to their chief judges. Nevertheless, as with the discomforting aspects of many of the Old Testament prophets, the people’s preferred solution was to silence them by putting them to death.
The Governor of the land retained that ability. We see in verse 21 that there are many chief judges, and that there is a Governor over all the land. At the beginning of the book of Alma, there was only the Chief Judge. That position appears to have become the Governor over all of the land, and it would be reasonable to assume that the individual cities and their associated land would also have chief judges.
The complaint comes to the Governor in Zarahemla. These prophets are to be tried according to the law.