Exchange Server 2007 doesn’t allow us to purge the disconnected mailbox. In order to remove one or multiple disconnected mailboxes we can be performing these steps:
Listing all disconnected mailboxes
Get-MailboxStatistics | where-object { $_.DisconnectDate -ne $null } | Select DisplayName,MailboxGuid
Removing a single entry
Remove-Mailbox -Database <Database-Name> -StoreMailboxIdentity <MailboxGuid> -confirm:$false
Removing all users at the same time
$users = Get-MailboxStatistics | where-object { $_.DisconnectDate -ne $null } | Select DisplayName,MailboxGuid