2002 Saturn L300 thermostat housing

I recently had my thermostat housing replaced at my mechanics. They charged me for 4 hours labor, plus the diagnostic. When I drove the car the next day, heat came pouring out of the vents even after I turned the fan off. Then I turned the temperature control knob all the way off, still heat coming out. Then I turned the A/C on and heat continued to come out of the vents. Any suggestions on what this might be and/or how to deal with the auto shop on this. I've been going there for ten years and this is the first problem I've had.