• Grant FullAccess to the mailbox you need to blank
Get-Mailbox "target-username" | Add-MailboxPermission -AccessRights FullAccess -User "my-username"
  • Delete the content
Get-Mailbox | Export-Mailbox -DeleteContent
  • Remove FullAccess to mailboxes
Get-Mailbox "target-username" | Remove-MailboxPermission -AccessRights FullAccess -User "my-username"

From: http://community.spiceworks.com/how_to/show/892-delete-emails-in-exchange-2007