I use this as a daily scheduled task to hide users disabled in AD from the Global Address List
$mailboxes = get-user | where {$_.UserAccountControl -like '*AccountDisabled*' -and $_.RecipientType -eq 'UserMailbox' } | get-mailbox | where {$_.HiddenFromAddressListsEnabled -eq $false}
foreach ($mailbox in $mailboxes) { Set-Mailbox -HiddenFromAddressListsEnabled $true -Identity $mailbox }