It's a simple conditional :)
In your showthread template, try this:
Find:
<if condition="$show['quickreply']">
<!-- quick reply -->
<form action="newreply.php" method="post" name="vbform" onsubmit="return qr_prepare_submit(this, $vboptions[postminchars]);" id="qrform">
<table class="tborder" cellpadding="$stylevar[cellpadding]" cellspacing="$stylevar[cellspacing]" border="0" width="100%" align="center">
<thead>
<tr>
<td class="tcat" colspan="2">
<a style="float:$stylevar[right]" href="#top" onclick="return <if condition="$show['wysiwyg'] == 2 AND !is_browser('ie')">editor_loaded ? false : </if> toggle_collapse('quickreply');"><img id="collapseimg_quickreply" src="$stylevar[imgdir_button]/collapse_tcat$vbcollapse[collapseimg_quickreply].gif" alt="" border="0" /></a>
$vbphrase[quick_reply]
</td>
</tr>
</thead>
<tbody id="qr_error_tbody" style="display:none">
<tr>
<td class="thead">$vbphrase[errors_occured_when_submitted]</td>
</tr>
<tr>
<td class="alt1" id="qr_error_td"></td>
</tr>
<tr>
<td class="tfoot" align="center"><span class="smallfont"><a href="#" onclick="return qr_hide_errors()">$vbphrase[okay]</a></span></td>
</tr>
</tbody>
<tbody id="collapseobj_quickreply" style="$vbcollapse[collapseobj_quickreply]">
<tr>
<td class="panelsurround" align="center">
<div class="panel">
<table cellpadding="0" cellspacing="0" border="0" align="center">
<tr>
<td class="smallfont" align="$stylevar[left]">$vbphrase[message]:</td>
</tr>
<tr>
<td id="$editorid" class="vBulletin_editor" align="$stylevar[left]">$messagearea</td>
</tr>
<tr>
<td align="$stylevar[left]">
<fieldset class="fieldset" style="margin:$stylevar[formspacer]px 0px 0px 0px">
<legend>$vbphrase[options]</legend>
<div>
<if condition="$bbuserinfo['signature']">
<div style="float:$stylevar[right]"><label for="cb_signature"><input type="checkbox" name="signature" value="1" id="cb_signature" tabindex="5" checked="checked" />$vbphrase[show_your_signature]</label></div>
</if>
<label for="qr_quickreply"><input type="checkbox" name="quickreply" value="1" id="qr_quickreply" accesskey="w" tabindex="4" />$vbphrase[quote_message_in_reply]</label>
</div>
</fieldset>
</td>
</tr>
</table>
</div>
<div style="margin-top:$stylevar[cellpadding]px">
<input type="hidden" name="fromquickreply" value="1" />
<input type="hidden" name="s" value="$session[sessionhash]" />
<input type="hidden" name="do" value="postreply" />
<input type="hidden" name="t" value="$threadid" id="qr_threadid" />
<input type="hidden" name="p" value="$qrpostid" id="qr_postid" />
<input type="hidden" name="parseurl" value="1" />
<input type="hidden" name="clickedelm" value="" />
<input type="submit" class="button" value="$vbphrase[post_quick_reply]" accesskey="s" title="(Alt + S)" name="sbutton" tabindex="2" id="qr_submit" onclick="this.form.clickedelm.value = this.value" />
<input type="submit" class="button" value="$vbphrase[go_advanced]" accesskey="x" title="(Alt + X)" name="preview" tabindex="3" id="qr_preview" onclick="this.form.clickedelm.value = this.value" />
</div>
<div align="center" id="qr_posting_msg" style="display:none; margin-top:6px">
<strong>$vbphrase[posting_quick_reply_please_wait]</strong>
</div>
</td>
</tr>
</tbody>
</table>
</form>
<if condition="!is_browser('ie') AND $show['wysiwyg']">
<!-- Mozilla work around for focusing on QR in WYSIWYG mode -->
<div id="qr_scroll"></div>
</if>
<br />
<script type="text/javascript" src="clientscript/vbulletin_quick_reply.js"></script>
<!-- end quick reply -->
</if>
Wrap it with this:
<if condition="$show['closethread']"></if>
I haven't tested this, we may need to tinker a bit.