postfix block mail to a domain only for a specific destination domain

You can do this using smtpd_restriction_classes in main.cf like this:

smtpd_recipient_restrictions = 
    ...
    check_recipient_access hash:/etc/postfix/recipient_access
    ...

smtpd_restriction_classes = tocustomerx
tocustomerx = check_sender_access hash:/etc/postfix/customer_x_sender_access

In /etc/postfix/recipient_access put something like this:

dom-customer.com tocustomerx

And in /etc/postfix/customer_x_sender_access

dom-xyz.com REJECT

This tells postfix that mails to your customers domain belong to class tocustomerx and that for the class it should check /etc/postfix/customer_x_sender_access for the sender access.