The exact same thing happened to me... easy fix. There is a flap that mixes your cold air and hot air that is controlled by a motor and a push arm down by your right leg when sitting in the driver's seat. If the plastic clip that holds the control arm to the motor breaks (which is what happened to me) then you get constant heat ALL the time.
Here is the fix:
http://www.clarks-garage.com/shop-manual/hvac-01.htm
Scroll all the way to the bottom till you see this:
