You are using Email to Case Premium and would like to prevent inbound emails from specific email addresses (spam addresses) from creating cases in your Salesforce Org.

Out of the box, Email to Case Premium (E2CP) will never intentionally prevent an email from being successfully processed and creating a case. However, there are a few different ways to approach this type of situation.

1. (Recommended) Ask your IT / Email Server Administrator to prevent specific emails from forwarding into Salesforce for the problematic addresses at the Mail Server level. Also, use a Domain Name System-based Blackhole List with your email server to avoid having to update this list manually.

2. The 'Block Email List' setting in the E2CP Setup Wizard allows you to manually specify email addresses and/or domains that Email to Case Premium should automatically block from creating and updating cases in Salesforce. 

3. As a Salesforce Administrator, you may consider taking the route of creating a validation rule to prevent case creation.

To create a validation rule to handle this situation, please follow the steps outlined below.

• Navigate to Setup > Customize > Cases > Validation Rules.
• Click the New button and specify a name for your new rule, such as "Blocked Email Addresses."
• Specify a Description (Optional).
• For the Error Condition Formula, specify SuppliedEmail = 'Enter the Problematic Address Here."
  • For multiple addresses, add a comma after the first address and specify each additional address in single quotes.
• Specify an Error Message for when the email is blocked from being processed. An email with the error will be sent to the Error Routing Address specified under Setup > Develop > Email Services > Email to Case Premium.
• Click Save, and you're all set. The final rule should look similar to the following screenshot.