There is no direct block against an email address that I am aware of (except to only receiving emails from known profile records) here are a couple of alternatives:
- It might be possible to block the accounts at the exchange level from the Service Desk mail box
- Create a quick Action to close the records as spam
- Add a Trigger Business Rule to close (or delete) records from the unwanted email addresses
You could create a required business rule based on Source = "Email" that would not be filled at time when email is processed. The Incident record would fail to create because the required field is not filled in.
Alternatively, you could preempt the Email Listener by using an exchange mailbox rule to move the email before the listener captures it. While that is not maintained in HEAT it is possible to allow users to create these rules if they have access to the mailbox.
I'm not sure how your environment operates so I'll just harrowing it out there. We just launched and the business is still a bit apprehensive so they are still monitoring their inbox for inconsistencies right now.