Here are the simple steps which help to block your iCloud mails.
SIGN IN with your iCloud ID.⮞ Click on Mail.
⮞Click or hold on the User want to block.⮞ Click on the settings icon to appear on the left bottom of the screen.
⮞Click on rules.⮞ Click on ADD Rules.⮞ Paste user id.
⮞Now Click on “Move to Trash”. Click on Done!