This article explains how to create a rule to block specific email addresses in iCloud Mail. Any blocked emails will automatically go to the Trash, where they'll be quickly deleted so you never have to view or open them again.

To block a sender and send his/her messages to the Trash:

Sign in to iCloud Mail and select an email from the sender you want to block. You don't need to open the email.

To view your mailboxes and no mailboxes are displayed, select the right arrow (Show Mailboxes) in the upper left corner.