> That being said it's usually mandatory to keep them at least half-open considering that the heat is shared with other apartments.

Yeah that was kinda the thing. She could technically close it but then the neighbours would complain.

I would hate that as most of the time I use no heating (or cooling) at all. Also, I wouldn't like to pay for it if I don't even use it.

PS: I live in a nicely mild-climated mediterranean coastal city so I can imagine "not using the heater" is not really a done thing if you live in Finland :)