Dcpromo is the Windows 2000 and Windows Server 2003 GUI interface for promoting a server to the role of being a Domain Controller, and if is already a DC, then dcpromo will be the tool to use to demote it back to being a member server. If you run Dcpromo on an existing DC to demote it and it fails that you can Dcpromo with the /forceremoval switch (The big Hammer), which tells the process to ignore errors. With /forceremoval, an administrator can forcibly remove Active Directory and roll back the system without having to contact or replicate any locally held changes to another DC in the forest.
After you use the dcpromo /forceremoval command, all the remaining metadata for the demoted DC is not deleted on the surviving domain controllers, and therefore you must manually remove it by using the NTDSUTIL command.
For more information please read:
http://www.petri.co.il/forcibly_removing_active_directoy_from_dc.htm
http://www.petri.co.il/delete_failed_dcs_from_ad.htm
