Thinking about this... yes I agree they don't like huge crosses all over the place, so why not just leave frostfreezitude out of the list of tick boxes, rather than put "n/a"?
They leave other stuff off apparently at random so why not? Complete absence of the tick box then implies it doesn't have that feature.
Or, use a small cross?
Or, leave the tick box blank. That might even be less ambiguous than "n/a".
There is a danger here that one might be considered to be taking this a little too seriously.