@Groove:
usually with "hard" it is meant "hardware", but I mean HARD, totally, complete. I mean "re-set" in the literally meaning of the therm: re-set the condition of Arduino as it exited from factory.
@robtillaart:
It isn't a mission impossible.
Simply an extra button that, when pressed, activate a portion of code into the bootloader of Arduino that erases (fill with 00) the portion of Flash Ram usually occupied by the user code or maybe reload the simple blinking code.