Find the code block below and comment it out or remove it in newthread template
<vb:if condition="$show['tag_option']">
<div class="blockrow">
<label for="tagpopup_ctrl" class="full">{vb:rawphrase tags}:</label>
<div id="tagpopup" class="popupmenu nomouseover noclick nohovermenu">
<input type="text" class="primary full textbox popupctrl" name="taglist" id="tagpopup_ctrl" value="{vb:raw newpost.taglist}" tabindex="1" />
<div id="tagpopup_body" class="popupbody">
</div>
</div>
<script type="text/javascript" src="clientscript/vbulletin_ajax_suggest.js?v={vb:raw vboptions.simpleversion}"></script>
<script type="text/javascript">
<!--
tag_add_comp = new vB_AJAX_TagSuggest('tag_add_comp', 'tagpopup_ctrl', 'tagpopup');
tag_add_comp.allow_multiple = true;
tag_add_comp.set_delimiters('{vb:raw tag_delimiters}');
//-->
</script>
<p class="singledescription">{vb:rawphrase separate_tags_using_comma}<vb:if condition="$show['tags_remain']"> {vb:rawphrase you_may_add_x_tags_to_thread, {vb:raw tags_remain}}</vb:if></p>
</div>
</vb:if>