Civil action
The first Kaong Zon Emperor, Le Zeing, decrees that all mages must be registered with the imperial government in order to practice magic legally, on pain of death for disobedience. This was to consolidate power as mage-run kingdoms were Le Zeing's main opposition to Gui unification during the Zer Ban Yu.
While this does not decrease the number of mages at all, it does cause a near collapse of many industries which rely on or were built around magic. Many mages were approved, but any who practiced magic that Le Zeing disliked were executed as soon as they appeared to be registered. It took 2 centuries to recover from the economic downturn this caused.