How can I stop sending NDR "Undelivered Mail Returned to Sender"

Server has been added to a backscatter database. So I would like to disable the sending of NDR emails, specifically "Undelivered Mail Returned to Sender"

I have read various things online, and neither of these work:

1) Adding this to /etc/postfix/master.cf

bounce    unix  -       -       n       -       0       discard

1) Adding this to /etc/postfix/header_checks doesn't work (and header_checks is enabled in main.cf)

So how does one just stop sending these please, so we can not be added to backscatter databases?

Thanks