A reservation booked that cant be assigned because we don't have the space

REMINDER : DO NOT POST CANCELLATION REQUESTS ON THE PARTNER HUB

Cancel it yourself with the guest directly by either:

1. Direct Guest to logon t obooking.com and cancel it themselves there.  
2. use the request cancel on their reservation details page, after sending them a message of cancellation intent, then  choose option 1, system notifies guest by email, guest must click on a confirm link, system updates the reservation to cancelled. If they prepaid BdC refund in 100% is processed.

PS : finish your profile by adding listing link

How -To : How & Why Add your Listing to your Partner Community Profile | Booking.com for Partners

Reminder Partner Support will never see this.

Posting account or action requests here will result in nothing.

 Message or Call BdC PartnerSupport ***

1. In the extranet, go to the ‘Inbox’ tab.
2. Select ‘Booking.com messages’.
3. Right Pane - Click on ‘See Contact Options’. 
4. Choose Account
5. Now choose Other
6. Then at bottom , See all contact options, and click Message or Call
7. As there is no one number for all partners, the number for your nearest BdC Partner Support is then revealed.

*** If no extranet access use the main public lines from here 

Kind Regards

I suspect you are also listed on more than one OTA and not using a 3rd party channel manager service such as syncbnb.com

The BdC calendar sync feature is both limited and not real time instant, so it will never sync bi-directionally in real time and result in double booking//overbooking.