How do I delete an alias to my Apple ID?

Trying to deal with @me.com addresses an be a bit challenging. I suggest you pose the issue to Apple itself:

Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support

See this article about managing email aliases Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support