Для одного:
Get-MailboxStatistics | where-object { $_.DisconnectDate -ne $null } | Select DisplayName,MailboxGuid
Remove-Mailbox -Database <Database-Name> -StoreMailboxIdentity <MailboxGuid> -confirm:$false
Для пачки:
$users = Get-MailboxStatistics | where-object { $_.DisconnectDate -ne $null } | Select DisplayName,MailboxGuid,Database
$users | ForEach { Remove-Mailbox -Database $_.Database -StoreMailboxIdentity $_.MailboxGuid -confirm:$false }