Remove an On-Prem Firewall Management Center from CDO
Use the following procedure to remove an on-prem management center and it's registered devices from CDO:
In the navigation pane, click Inventory.
Click the Devices tab.
Click the On-Prem FMC tab and select the on-prem management center you want to remove.
In the Device Actions pane located to the right, click Remove FMC and its managed devices.
Click OK to confirm that you want to remove the on-prem management center and its managed devices from your tenant.
Refresh your browser to seen an updates list of available devices.