Removing disconnected mailboxes in Exchange Server 2007
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