Webmin uses the Postfix directive "check_client_access" in "smtpd_client_restrictions" in the "SMTP Client Restrictions" dialog.
Problem is: To block incoming mail based on envelope sender address, you need "check_sender_access" in "smtpd_sender_restrictions". In "client_access", only rejection based on IP address and hostnames is possible.
The Webmin help text says though that I can also put email addresses in the client_access hash map, which does not work -- I just tested that.
So Webmin should have an option to edit a sender_access map, and the proper map should be used for IP addresses, hostnames and email addresses.