I'm with Taha.
The valve we already see there should already be able to stop the flow (when your "warm pull" knob is pushed all the way home).
If it doesn't, maybe it's time to replace it rather than fit something extra to do the job it should be doing.
Well, even if it still dribbles a bit of hot coolant into your heater core with your "Warm Pull" knob pushed all the way in, it still won't noticeably heat your cabin (if my experience is anything to go by).
And I find that with my fan knob pushed in (ie. no fan), my heater core never puts any noticeable heat into the cabin even when my "Warm Pull" is pulled out (unlike some vehicles where you have some natural air flow through the core from vehicle-movement/air-ram-effect).