The stock heater control valve comes in two varieties: early (with a bent-rod actuator) and late with a rectangular steel actuator. They both work about the same, that is, they will allow hot water to slightly bypass the valve. The reason is, the leverage from the valve actuator and the dash levers differ. So you have the choice of adjusting the cable controls all the way OFF but only part-way on when you want heat/defrost, or all the way ON but only part-way off when you don't want heat. Which then "leaks".
A pair of manual 1/4-turn ball-valve shutoffs somewhere in the heater lines are common and is the simplest fix; manual shut-off valves are required for track events for safety as the short heater hose below the dash is usually neglected and many have blown at high rpms, scalding driver or passenger's feet, and markedly improves A/C performance since leaking hot water thru the heater core degrades A/C performance.
Track req. is for two valves to ensure the pressure side (with the rubber hose) is positively shut off because its difficult to guarantee which line is shut off without pulling the firewall access door during Tech Inspection. That's also why MANUAL valves are required; electric valves may work fine on the street but wires neatly cabled prove nothing to a harried Tech Inspector.