Doubt it is a feature in vB.. I tried helping you with the following variable combination:
PHP Code:
if (is_member_of($vbulletin->userinfo,xx))
$show['quickreply'] = false;
if ($_REQUEST['do'] == 'newreply')
With the threadID variable missing, someone (or you) should be able to create a block for it.. I just didn't succeed yet. But you are a coder, bet you can figure it out! Still new to all of this and learning.