There is a water valve mounted on the firewall that might be usable to turn off the water to the heater core. It's mounted on the firewall behind the engine. On left-hand drive cars, it's on the driver's side next to the brake master cylinder. It normally turns off the water to the heater at low RPM.
You can go to RealOEM.com to find a picture of it for your model.
I'm not sure, but maybe if you disconnected the wire to the valve, it would be "off" and you can enjoy your car for the rest of the summer. Of course you can re-route the hoses to bypass the heater core, but it is really tight back there and not much fun to work on the hoses.