Please note that there is a difference between "Closing the room in Booking.com" and just "Blocking the room in HostelSystem".
Room which is closed on Booking.com is not affected by any availability changes in Hostelsystem.
For instance, if you close the room for January, doesn't matter if you have it blocked, booked or available in HostelSystem, as none of the changes will influence the room in Extranet.
In order to close the room in Booking.com you must go to MANAGE / ONLINE PORTALS / BOOKING.COM / SYNCHRONIZE tab and run the CLOSE command for the room and period of time when the room should be closed.
If you want to open the room, you must run the NOT CLOSE command for the room in question and the period of time you want to have the room open.
On the contrary, if you your room is open in Extranet (not closed), and you block the room in HostelSystem, then the room in Extranet will stay open, but with availability "0".