The only thing required to unregister a machine is to run the unregister command. This removes the system's entry from the subscription service, unsubscribes it from any subscriptions, and, locally, deletes its identity and entitlement certificates.

In the Red Hat Subscription Manager GUI, there is an Unregister button at the top of the window in the Tools area.

From the command line, this requires only the unregister.

[root@server1 ~]# subscription-manager unregister