You can delete the domain following the steps in this guide: https://learn.microsoft.com/en-us/azure/active-directory/enterprise-users/domains-manage
The original domain like contoso.onmicrosoft.com can't be removed, and any top-level domain that has subdomains associated with it cannot be removed until the subdomains have been removed. For other custom domain names, you must change or delete any related resources in your Azure AD directory before you can delete the custom domain name. Only a Global Administrator can manage domains in Azure AD, so make sure you have those permissions.
To delete a custom domain name, you must first ensure that no resources in your directory have dependencies related to the domain name. You can't delete a domain name from your directory if:
- Any user has a user name, email address, or proxy address that includes the domain name.
- Any group has an email address or proxy address that includes the domain name.
- Any application in your Azure AD has an app ID URI that includes the domain name.
To delete a domain:
1.) Sign into the Azure classic portal using an account with global admin privileges for that directory.
2.) Open your directory, and select Domains.
3.) Select the domain and click Delete.
See related thread.