Add "Disable outbound mail" option to Global Settings >> Outbound Mail (SMTP) configuration form. Clicking this option will clear all smtp settings on the form and bypass the field requirements.
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If one wants to disable the "Sent e-mail to contacts" feature in the action drop down on the contact search results screen, there's currently no (easy) way to do this. It should be possible to remove all values from the SMTP settings screen, but currently that option is blocked by the input validation on the SMTP host and SMTP port fields, e.g., trying to remove the values storied there returns an error.
It should be possible to turn off this feature so that an administrator can disallow the direct e-mails of contacts through CiviCRM.