How can you forcibly remove AD from a server, and what do you do later? ? Can I get user passwords from the AD database?

Dcpromo /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. Reboot the server then 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.

In the event that the NTDS Settings object is not removed correctly you can use the Ntdsutil.exe utility to manually remove the NTDS Settings object. You will need the following tool: Ntdsutil.exe, Active Directory Sites and Services, Active Directory Users and Computers.