Fixing vertical scrolling

Hi,

I have a small issue where a character walks down from the top floor to the lower floor, at which point I'd like the vertical scrolling to be fixed until they go back up the stairs again.

There is an action to go upstairs and downstairs, so I don't need to use the character's position to switch this on/off necessarily. I've tried tweaking a few different properties like GameScrollDirectionVertical and GameScrollPosition but I'm having no luck.