My best guess about how Ermynn "erased magecraft completely" is that the word choice that is very deliberate. They erased the magecraft. They erased the knowledge of how to practice it, so that even if someone were born with the Veins of Guidance, there would be no one to teach how to use it.
Going back to the history in previous chapters, there was a kingdom of magic that fell, and somehow that fall led to the discrimination of mages in Lavarta. There may have been a similar logic in Ermynn that led to the erasure of how to perform magic, which, based on things established in the story, likely means there are plenty of magic related problems within Ermynn.
A notable one that comes to mind: how do people born with Veins of Guidance in the country manage to keep their latent power under control? Do they not go berserk? Perhaps it is an issue in Lavarta with untrained mages facing discrimination that they lash out against, but surely untrained mages in Ermynn also feel strong, negative emotions sometimes.