Make sure that DC is offline, and will stay offline first.
After that you can delete the computer account from the ADUC's console, it will prompt you a message, that if you forcelly remove it, say yes. It will remove reference to it from ntdsutil.
You will need to clean the DNS's zone afterhand, as some entry might still reference it.
Make sure your DHCP no longer give that DC IP.
Except that, it should be clean after.