How to stop Postfix from accepting mail to [email protected]

Just remove $myhostname from mydestination and restart postfix

You mention smtp_recipient_restrictions, have you actually tried adding the line check_recipient_access hash:/etc/postfix/recipient_access?

Place it as the first line before reject_unknown_recipient_domain, then in /etc/postfix/recipient_access add the line:

[email protected] REJECT

(don't forget to postmap /etc/postfix/recipient_access)

BTW, the postmaster address should always be available, but I have never seen an RFC that said that root had to be available, although I haven't read every single RFC that relates to SMTP.