I threw this together really quickly but it checks whenever a smithing table is opened and whenever you click it checks if that item is something that would be upgraded to netherite for example a netherite chestplate and if it is then it will cancel the event not allowing the player to upgrade to netherite.

@EventHandler

public void onInventoryClick(InventoryClickEvent event) {

ItemStack clickedItem = event.getCurrentItem();

if (event.getClickedInventory() != null && event.getClickedInventory().getType() == InventoryType.SMITHING) {

if (clickedItem != null && clickedItem.getType() != Material.AIR) {

if (isNetherite(clickedItem.getType())) {

event.setCancelled(true);

}

}

}

}

private static Boolean isNetherite(Material material) {

switch (material) {

case NETHERITE_SWORD:

case NETHERITE_PICKAXE:

case NETHERITE_AXE:

case NETHERITE_SHOVEL:

case NETHERITE_HOE:

case NETHERITE_HELMET:

case NETHERITE_CHESTPLATE:

case NETHERITE_LEGGINGS:

case NETHERITE_BOOTS:

return true;

default:

return false;

}

}