@josepp hi you can add server hostname to postfix reject list in order to stop receiving from them.
Like in :
/etc/postfix/sender_access
Add one of below which fits you i.e. you don't want mail from those sans or from particular user matches the name.
domain.com REJECT
sub.domain.com REJECT
user@ REJECT
now run postmap:
postmap /etc/postfix/sender_access
Ensure you added the reject config or present in /etc/postfix/main.cf
smtpd_recipient_restrictions = check_sender_access hash:/etc/postfix/sender_access
At last restart postfix service.