Hello Matt_Ma,
Thanks for posting in the Apple Support Communities. We're happy to look into this with you. We understand that you're looking for options to disable or delete iCloud/Apple ID alias. Have you tried following the actions provided in article, Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support? The steps provided under section: Turn off, turn on or delete an alias, should help with this.
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. While 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. In Mail on iCloud.com, click at the top of the Mailboxes list, then choose Preferences.
2. Click Accounts, select an iCloud Mail alias in the list, then do any of the following:
- Turn off an alias: Select “Disable alias.”
- Turn on an alias: Deselect “Disable alias.”
- Delete an alias: Click “Delete alias,” then click Delete.
3. Click Done.
Note: You can’t delete or turn off a primary iCloud Mail address. You can’t use an alias to create a separate Apple ID, and an email alias can’t be converted to a primary iCloud Mail address.
If you continue to have issues with the process, let us know and we'll be happy to research additional options that may be helpful or contact Apple Support for additional guidance at Get Support.
Best regards.