The system is set up this way intentionally. The failsafe mode is supposed to be full heat with air up on the windshield. MB is thinking in terms of a failure on a cold snowy day and trying to maintain a clear windshield. They figure if the failure occurs in July in Florida for example you can open the windows. You can't just unplug the heater valve, as there is no power going to it now, this is part of the failsafe; no power means heater valve is open, not closed. You could maybe get something to pinch off the heater hose; either side should reduce the heater core temp after a few minutes, the heat should dissipate. The easiest place would probably be in the area of the aux cooling pump on the right side of the engine down low in the engine compartment. The best way would be to get one of these ratcheting type pliers that are intended to pinch off hoses, this would be less harmful to the hose then using say a vice-grips, as the special pinch-off pliers have smooth jaws so shouldn't do any damage to the rubber like a jagged vice-grips jaws.
Hope this helps....