Yeah Zodiark was just an Aetheric shield - so he could have been allowed to wander the moon (not causing trouble- since no one is there)... and the Ancients could have put up Aetheric Barriers around the city. It might not have been a permanent solution, but certainly would have bought time, especially since they could have actually then known what was happening...."Hey guys stop killing yourselves to feed Zodiark- his Aetheric energy is the shield... let's try building an Aetheric barrier instead of feeding ourselves to this guy!"