If you want the heater permanently off, you could just dissconnect the heater hose, plugging up the end it came from. That way no heat gets in there at all. Or usually there are flat metal rods that go into the unit that acts as a tap letting hot water from the radiator system into the unit allowing to to heat up (sort of like a heating radiator) if the tap is closed off no hot water gets through, but hot water still goes through the pipe up until that tap, and in sunner you can feel it sometimes.

I hope some of this has helped
Cheers
Damo