Unable to delete registered iCloud accounts with custom domain

Months ago I bought a custom domain. It seems like someone has registered an iCloud Account with my custom domain without my permission to do so. Now he can use my custom domain and fake my person.

How can this email account or iCloud account be deleted and I am gaining access again to this address via catchAll?

I tried to contact Apple via Postal, E-Mail, Customer Services and nobody can help me. I am in charge of this domain and the only person who wants to use and administrate this.

Example:

I am registered with abc@icloud.com. It has the custom domain example.com. Somebody registered himself with contact@example.com and is getting now all of my emails going to contact@example.com instead I am receiving this via catchAll. When I want to register contact@example.com to abc@icloud.com via Settings, it says, a iCloud User has already been registered, but is no family member.

I really don't want to switch to a third party email company, I just want to handle all registered accounts and get my from business site needed email address back. How can I solve this?