Although the iOS app doesn't have an easy blocking option, you can still mark an email someone sends as spam. Obviously, we're talking about email, but don't forget that you can also block calls and contacts and messages on your iPhone or iPad. You may want to make sure someone's email address is also on the contact card. Make sure the email address you want to block is in the Contacts app. If the address is not in the Contacts app, add it to an existing contact or create a new contact card. When the email address is in Contacts, open the Settings app. Conversation Options, tap the Blocked Senders option. Choose what to do with messages from blocked senders.

See also - https://emailspedia.com/how-to-block-email-on-your-iphone/