One option I am looking at, Setup > Company > Enable Features > CRM > Marketing Option to Capture Email Replies. If I uncheck this then NetSuite will not capture Replies at all, would be nice to just block attachments.

Help Section for Capture Email Replies: Check this box to track all replies to email that was originally sent from NetSuite. For example, if a support representative responds to a case record by email and the customer replies to the email message, the response is automatically saved on the case record as well as the customer's record. This feature also works with email sent from the Messages subtab of relationship records. All replies to the original email are saved on the Messages subtab.