It ain't pretty, or recommended for the long term, but something like this
should slow it down

    /etc/postfix/main.cf
        header_checks = pcre:/etc/postfix/header_checks.pcre

    /etc/postfix/header_checks.pcre
        IF /^(To|From|Cc|Reply-To|Sender|Return-Path): /
        /@qq\.com/i   REJECT
        ENDIF

​(check the syntax!)​

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