Lock ceruleum tanks to free company credits. Disable trade.
This will distinguish between the shell FCs and legit FCs.
Lock the magitek repairs in crafting too. Do not make them tradable.
Edit: Limit free company credits to FC activities (duties, crafting, gathering, etc similar in nature)
Ofc some of these house hoarders will still go out of their way but limiting ceruleum tanks will be a strong blow against it.