How do I turn the heater off in a 1968 Buick Riviera?

I recently bought a 1968 Buick Rivera and can't seem to figure out how to turn the heater off. The heater continues to blow out of the defrost vents. I tried switching the controls off, but nothing happens when I try to turn it off. I can get temperature change when I switch from cold to hot but nothing else. Somebody at Auto zone said that the car was climate controlled but I can't get it to turn off. Any help would be appreciated Thanks|||Some rivieras had a thing called full flow ventilation system,it was in operation at all times when the engine is operating, even when the car is standing still , this works by the blower motor runs constantly at low speeds,there was no off position on the blower switch,,But i thought this was not used until the boatail 1971--73,but it may have started on from 67 68 the first of the 430 ci engine,.anyway buick did have alot of complates about this at the time.I would consider mounting a under dash on off switch,after finding the correct wire to eliminate it when required.