What quick reply button are you talking about? I see Post Reply and I see Post Quick Reply. The second is required in order to post from the Quick Reply box which you say you want to keep. Or are you talking about the little one in each post? You would just remove the code for that in your postbit or postbit_legacy template. I believe it is a line like this:
<if condition="$show['quickreply'] AND !$show['threadedmode']">
<a href="$post[replylink]" rel="nofollow" id="qr_$post[postid]" onclick="return false"><img src="$stylevar[imgdir_button]/quickreply.gif" alt="$vbphrase[quick_reply_to_this_message]" border="0" /></a>
</if>