Why does my radiator leak if it's not fully opened or closed?
I'm afraid your super is right. The supply valve must either be fully open or fully closed. If it is partially open, condensate cannot properly drain back to the boiler. It will collect in your radiator instead. How much heat is put out is regulated by the size of the vent valve. If the boiler works by central control, meaning the steam goes on and off for the whole building to regulate heat, you can regulate the heat put out by the radiator by getting a thermostatically controlled one pipe steam radiator vent valve, or simply getting a smaller vent valve.