If your attempt to use Dcpromo.exe to remove a domain fails, you could use Ntdsutil.exe to remove the domain metadata from the forest. When you try, you receive:
DsRemoveDsDomainW error 0x20ab.
The trust relationship is broken.
To workaround this problem, remove the following references from the Active Directory Database using adsiedit.msc or lpd.exe:
NOTE: DomainName is the name of the domain.