for anyone looking to do same, i have managed a bit of a workaround. For us we use cpanel for the hosting backend. and have setup a simple email rule on our "helpdesk" email account. A rule that says if subject does not contain "[#0"
then "fail with message". then add a helpful message. it's not ideal since the user gets a formal email bounce but the friendly message within does help.
and it's clunky given the subject check on "[#0" effectively checks if the email coming in is a reply to an existing call or a blanket new email.
once it fails it gets removed and the pipe never runs, so OST never collects the message.
This is certainly a feature i would like to see in future releases. and should be not too hard to do something like above in the engine. and a tick box in the admin page for "reject new calls over email"
cheers
Gus