Is there any way I can disable Workflow email notifications completely, from within the CMS itself?
As in; without say disabling SMTP settings.
Well the first thought that comes to mind is the deletion of the E-mail Action Item, found in the System/Workflows area of the content tree. It would appear under the workflow that you currently have running and under specific workflow states.
You could also optionally just remove the To field, so it wouldn't send it anywhere, but that might be what you were referring to above as an SMTP setting.
Another thought, some people extend the Email Action with custom logic. If you have done this, you could also create a setting that wraps the logic, that if the setting is set to false (not to send), it would not send these e-mails. That may be the best option especially if you want to disable e-mails only for specific environments.