How can I delete an iCloud alias

See the following from Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support

Turn off, turn on, or delete an alias

You can turn an email alias on or off at any time. You can also delete email aliases that you no longer want to use. Turning off an email alias is temporary; deleting it is permanent. When an alias is turned off or deleted, email sent to that alias is returned to the sender.

1. Go to icloud.com/mail, then sign in to your Apple Account (if necessary).
2. Click  at the top of the Mailboxes list, then choose Settings.
3. Click Account, select an iCloud Mail alias in the list, then do any of the following:

• • Turn off an alias: Select “Disable alias,” then click Save Changes.
  • Turn on an alias: Deselect “Disable alias,” then click Save Changes.
• Delete an alias: Click “Delete alias,” then click Delete.
  • Note: If you have three email aliases and you delete one, you must wait 7 days before you can create a new one.

I would recommend: Contact - Official Apple Support