Yes, you can delete those users in AD if you like. Unity will never delete a user object from the directory (we'll remove contact for Bridge users if configured to do so, however). We didn't want to get into situations where a Unity admin blew away legitimate users in AD at UM sites - much too dangerous. So when you delete a subscriber in Unity we only clean the properties off the AD object and leave it there.
You can check out tools such as Bulk Subscriber Delete on the www.CiscoUnityTools.com web site that _will_ remove the AD objects if you like - this is normally used when removing large numbers of users en mass.
