Helped me remove iCloud mail from iPhone 7 plus
Remove iCloud from iPhone 7 Plus
Question marked as
Are you trying to remove an iCloud Apple ID or the iCloud email address in Mail?
If the email address in Mail, have you tried going to Settings/Passwords & Accounts/Accounts/iCloud and sliding the Mail button to the left (no green showing).
If the ID, have you signed out of the places in the article below?
What to do after you change your Apple ID or password - Apple Support