Older steam boiler wiring repair question

I just moved into a house that has a Utica PEG-187AS that was installed in 1983.  Some previous un-un-unskilled homeowner "fixed" the wiring leading to the boiler.



What I thought was the emergency switch at the top of the basement stairs, a pre-1955 or so switch box with fuses, turned out to only turn off the automatic water feeder.  With this switch off, the boiler will still fire.



I'm new to controls, though I understand the basics.  The boiler has a standing pilot, and I think I have a millivolt system, because with the main house breaker turned off, the thermostat can still start the boiler.



With this in mind, I would like to install an emergency switch that prevents the boiler from firing.  Lacking diagrams (the boiler manuals are long gone, and the Utica website doesn't have the older manuals), should I just interrupt the low voltage  thermostat circuit with a switch?