You don’t want users in your organization to send Out of Office Auto-replies to external recipients and also you don’t want the option to appear in Outlook 2007 like shown below
To disable/hide the option Outside My Organization in Out of Office Assistant in Outlook and in OWA use the following in Exchange Management Shell.
Set-Mailbox <mailboxid> -ExternalOofOptions InternalOnly
This can be changed per mailbox, if you want to be applied to all mailboxes there are many ways of approaching it. You can get mailboxes on a server and pipe it with Set-Mailbox with above settings Or the following will apply it on all mailboxes in an exchange organization.
Get-Mailbox –ResultSize Unlimited | Set-Mailbox -ExternalOofOptions InternalOnly
-ResultSize Unlimited is important otherwise it will change the setting for the first 1000 mailboxes.
For more details on Out of Office Assistant refer to the link below.