Yeah, there's a vacuum operated valve in line with one of the heater hoses to the heater core.
However, even with the valve closed you're going to get some heat transfer thru the valve and into the heater core.
I replaced the OEM vacuum valve with a custom ordered ASCO 12VDC solenoid valve, (after the replacement OEM unit I tried first eventually rusted up). Never got around to hooking the solenoid to a vacuum switch, 'cause I got plenty of heat from the transfer thru the closed valve, (and I live in a desert, so don't really need any extra heat anyway).
To stop the heat transfer completely, you have to take the heater core out of the circuit, and loop the inlet/outlet hoses together.
I've also heard of some people putting in throttle valves (barb-to-barb brass garden variety with the round faucet handle) in line to cut off the flow manually, in both lines. But you still get heat transfer (and stagnant coolant sitting in the heater core when they're closed).
Sorry...prolly TMI! lol