How can you forcibly remove AD from a server, and what do you do later?

Demote the server using dcpromo /forceremoval, then remove the metadata from Active directory using ndtsutil. There is no way to get user passwords from AD that I am aware of, but you should still be able to change them. 

Another way out too

Restart the DC is DSRM mode

a. Locate the following registry subkey:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\ProductOptions

b. In the right-pane, double-click ProductType.

c. Type ServerNT in the Value data box, and then click OK.

Restart the server in normal mode

its a member server now but AD entries are still there. Promote the server to a fake domain say ABC.com and then remove gracefully using DCpromo. 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 .