I don't know of a way to mark these since once a non registered user emails in and a ticket is created, they are added/registered to your user list. I'll let BOSS work out a way to mark them. I can offer a way to blacklist if you keep getting tickets from the same group of emails every time.
- Create a group for this "Blacklist"
- Search your user list for these unwanted email accounts and add them to the group
- Set a rule that states:
Condition: Requester Group is -- "Blacklist"
Action: Delete the Ticket