To remove a machine from a DNS domain:
Remove dns from the hosts line of the machine's nsswitch.conf file.
Remove the machine's /etc/resolv.conf file.
Delete the records for that machine from the primary server's hosts and hosts.rev files.
If the machine has CNAME records pointing to it, those CNAME records must also be deleted from the hosts file.
Set up replacements for services supported by the removed machine.
If the machine is a primary server, mail host, or host for any other necessary process or service, you must take whatever steps are necessary to set up some other machine to perform those services.