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