How to completely turn off Heat-only Mercury thermostat (honeywell, old)?

Here's my situation. A mudroom on the side of the house has a secondary dedicated hot water baseboard loop tied to the boiler, with its own circulating pump and thermostat, and ancient mercury honeywell mercury heat-only thermostat that is kept as low as possible, but has no "off switch" I can use to disable the thermostat altogether. I need a simple way to make the thermostat not call for heat anymore, without removing it, because the loop is drained and valves closed and if the room gets too cold, the circulating pump will start banging like the end of the world.

My only solution has been to place a small oil filled space heater on the lowest setting placed directly below the thermostat to "fool it", but what I really want is to leave the space unheated ( even when its very cold.)
I can only set the thermostat so low, if the room dips below the minimum temp setting, the circulating pump bangs and I have to run and shut the whole system down. There is a main power cut-off at the boiler, but it cuts off the whole schmoo, boiler, burner, main pump and the secondary pump, there is no separate cut-off for the secondary circulating pump.

So what I want to do is make it so the thermostat never calls for heat, no matter what the temp is, but I'm not sure how to safely disconnect the thermostat while leaving it in place. I don't want to have to worry about setting up a space heater anymore, not the least because it will trip the breakers in a space where I DO want an electric space heater most of the time. I just want to leave that room cold.

Thanks for any tips for temporary disabling the mercury thermostat