Option 1:

You can limit the email addresses or domains available for On-Demand Email-to-Case by entering them in this field, separated by commas in “Accept Email From”.

Option 2: 

Usually, email forwarding is used to forward emails from an email account to Salesforce Service Email Address to create Case in Salesforce. So, work with your Email Admin to check whether SPAM Filter can be created before forwarding the Emails to Salesforce Service Email Address.

GMAIL – https://support.google.com/mail/answer/6579?hl=en

Outlook:

https://docs.microsoft.com/en-us/microsoft-365/security/office-365-security/external-email-forwarding?view=o365-worldwide

https://docs.microsoft.com/en-us/answers/questions/135641/spam-filtering-before-redirecting-email.html