1. Login to a Domain Controller (Writable). You won’t be able to do this from the Adminstrator tools or any read-only domain controller. It needs to be done from a writable server.
2. Once in the server, start the Active Directory Users and Computers (ADUC) MMC from the Administrative Tools
3. Go to the Domain Controllers OU and right-click on the server that you want to remove.
4. Select “Delete” and then “Yes” to confirm the object deletion.
5. Check the “This Domain Controller is permanently offline and can no longer be demoted using the Active Directory Domain Services Installation Wizard (DCPROMO).” checkbox.
6. If the server is a Global Catalog, it will prompt you with another popup for permission to proceed. Click “Yes” to continue.
7. If all went well and no errors, the Domain Controller is removed along with all the metadata assosicated with it.