Originally Posted by MrM
..completely ignoring them would be a better option, but as I said, not strictly necessary.
Take a look at postfix restrictions parameters .. expecially at "reject_unknown_recipient_domain"
Requirements: the dns (mx or a) records of that provider domain must no longer exist.
If it suits your needs you will able ( modding your /etc/postfix/main.cf ) to do the trick in a better way ..you can avoid to receive the entire messages before dropping them (blocking them at smtp first dialog level) .. saving bandwith :-)