After the events in the flashback, the spirits seal off the Enchanted Forest so nobody can get in or out. They are described as "angry" but although their relations with the Northuldra are presumably better than the Arendelians, it seems that the Northuldra suffer the most from their wrath. Even though Runeard seems to have intended to cause direct harm to the spirits, so as to cause indirect harm to the Northuldra.
So why didn't the spirits seal off Arendelle instead? They did cause an earthquake there and temporarily made the place uninhabitable. Grand Pabbie seems to imply that worse is to come. Arendelle was a threat to the nature spirits while the Northuldra were not, so why didn't they contain the threat?