On my server I have made Platinum, gold, and uranium not spawn in asteroids, but the NUHOL detects it and the factory mines it anyway... Here is how I disable them:

[MySessionComponentDescriptor(MyUpdateOrder.BeforeSimulation)]
public class CustomDefinitions : MySessionComponentBase
{
private bool IsInit = false;
public override void BeforeStart()
{
MyDefinitionManager.Static.GetVoxelMaterialDefinition("Platinum_01").SpawnsInAsteroids = false;
MyDefinitionManager.Static.GetVoxelMaterialDefinition("Gold_01").SpawnsInAsteroids = false;
MyDefinitionManager.Static.GetVoxelMaterialDefinition("Uraninite_01").SpawnsInAsteroids = false;
}
}