[MegaTrain]

As discussed in the other thread about mutinies, your best bet for that is counter-intelligence.

The most common use for mothballing is to stop the payment of maintenance. If you have a large fleet just sitting around and waiting for a war to be declared, they use an incredible amount of resources in maintenance.
It'll cost you a tiny bit to mothball them, and a little bit more to unmothball them, but they don't cost you a thing in between the two.