If you remove the heater core you can just cut a bolt to plug the hose and clamp it in.
On my bus I use the rear heater in the winter, but obviously I don't want all that warm water running through the interior during the summer. So I have a valve on the two hoses that run through the interior heater cores. I just close it during the summer when I don't need the heat. I have never had a problem with overheating, the radiators on buses are huge.
Here is a picture that shows the valves, they are different because that is just what was around.