Chuckle - ancient issue I see coming full circle. Back in the day when heaters were an optional extra cost accessory and on up into at least the mid-1960s many if not all vehicles that had heaters operated with hot coolant circulating thru them all the time with only a thin plastic very poorly sealed blend door preventing or "trying" to prevent heat entering the cab when the heater was "off". That was superseded by mechanical (cable controlled) or electric or vacuum controlled coolant flow shutoff valves in the engine bay just in front of the firewall. That was then superseded by going back to the old hot all the time heater cores with the advent of better blend doors and blend door sealing.
Been thru all 3 of this cycle and I can assure you the 1st time thru the cycle things got quite toasty with the heater "off". At the time the solution was quite easy and it would work today - see
http://www.eurekaboy.com/f250/heaterhose.htm for the under $15 option. This option works fine, I've used it, to just shut the flow to the heater core off for warm months and turn it on for cold. Alternative see
http://www.ebay.com/itm/FORD-E-F-SERIES ... 0234390594 for a cable operated rig that could probably be installed for say $30 or less. I'll bet there are electric options.
For me though I'm not sure it's worth my time or $s as the AC works fine if it's hot enough and if it's not hot enough to run the AC I don't notice excessive heat from the system with the heater "off"