SharePoint Discussion board reply without previous posts content.

1. Open Discussion Board -> NewForm.aspx using SharePoint Designer
2. Paste following javascript code to bottom of the "MainContentPlaceHolder"

<script language="javascript" type="text/javascript">
    var d = document.getElementsByTagName("textarea"); 
    for( var i=0; i < d.length; ++i){ 
        if( d[i].id.indexOf('TextField') != 0 ){
        d[i].value = " ";
        } 
    }
</script>