I made the determination experimenting with a prior version of this circuit (in LTspice) that all I need do is remove the mutual coupling between the three central transformers and this will kill the overunity and, thus, turn OFF this circuit. So, the iron core could be removed, or else if they're air cored coils, then one could be slid out away from the other coil. And since these transformers are paired in parallel, then I don't know... Maybe not all of them need be uncoupled?

Secondly, it's probably remains a good idea to also ground out various points at the central core after the circuit "dies" just to be safe.