Generally, there are two reasons why obsolete users or groups can exist in the SharePoint Server 2010 user profile store:
-Obsolete users: The My Site cleanup timer job is not active. The User Profile Synchronization timer job marks for deletion users who have been deleted from the directory source. When the My Site cleanup job runs, it looks for all users marked for deletion and deletes their profiles. Respective My Sites are then assigned to the manager for the deleted user and an e-mail message notifies the manager of this deletion.
-Obsolete users and groups: Users and groups that were not imported by Profile Synchronization exist in the user profile store. This can occur, for example, if you upgraded from an earlier version of SharePoint Server and chose to only synchronize a subset of domains with SharePoint Server 2010.
Here is how you would do it in PowerShell: http://iedaddy.com/2012/02/sharepoint-2010user-information-lists-and-user-profile-cleanup/