How can I inform my guests that my flat is shared not private?

I had the exact same experience twice. I raised this to booking.com who removed the reviews as their monitoring department regarded them as unfair.

they said to me to message them to make changes to the description but don’t use the app only a desktop as the app isn’t very reliable 

Sorry I also had to trawl through the room facilities and amenities and remove kitchen etc as otherwise it will come up on the property as the rooms containing a kitchen.

the site is defo missing a few options here as it marginalises house shares by not providing options to state on the main booking page that a kitchen is included along with tv, washing machine etc, instead if you currently add those facilities it looks as if your private room has all of them in that private room and gives the impression that it is in fact a suite or a flat or something 

Booking.com might not be the best platform for shared accommodation. There will be AirBnB, where you can set your own descriptions that clearly state "Room in shared accommodation", or there will be more local websites where you can advertise your spare room.

You could insert text within each room photo that states if it is private or shared.

I added text to my stairway photo; so guests know they must walk down 16 steps to their private guestroom space.