Selecting a subset based on other records

Please see the attachment. I have a file containing customers and their assigned managers. A customer may have one or more manager. Some managers are from the Northern Region and other managers are from the Eastern region. I would like to retain the records for all customers who are assigned one or more of the Eastern managers exclusively--so not assigned to any of the Northern Managers. (I do have a separate list of the Northern Managers and the Eastern Managers.) Is there a simple approach that I'm overlooking?

Thank you!

Need a sample of your customer list (or dummy information in similar layout) in order to offer a suggestion...