This is definitely possible, and it's a really cool feature! Here's what you do:
Step 1: Close the rooms in myallocator
You can do this through Availability - Set Long Term - Closed - Set. It looks like below.
Step 2: Repeat for all desired rooms and dates
Repeat Step 1 for all your rooms, and all the dates you want to close to the outside channels.
Now, all of your closed rooms will be unavailable in the external channels (such as Booking.com, Expedia, etc). But they will still be available to book on your BookNow pages/widget.
NOTE: Not all channels support the close feature
Please refer to the channel features document to see which channels do not support sending the closed value. For those channels you will need to remove the rooms manually or work with the channel to not sell those rooms.