I have just posted a document explaining all the steps required to build an Operational Rule that will delete all devices of a device group. If you had a group that was dynamically created using a query with the criteria of “Last Update” = Less than 1 month. Then all devices that have not been updated for more than 30 days will be deleted thus freeing the admin from manually preforming this action.
Like all rules, please test thoroughly prior to deleting large number of devices.
Get the document here