[kh99]

You could just do something like this in a plugin using hook newthread_start:

if ($vbulletin->db->query_first("SELECT threadid FROM " . TABLE_PREFIX . "thread 
                                       WHERE postuserid = " . $vbulletin->userinfo['userid'] . " 
                                         AND forumid = " . $foruminfo['forumid'] . " 
                                         AND visible = 1 LIMIT 1"))
{
    eval(standard_error("Forum Rules allow one thread per user"));
}

or if you'd rather create a phrase for the error message:

   eval(standard_error(fetch_error(phrase_name)));

BTW, in general borbole's right, in many cases a mod wouldn't be this small so you wouldn't be likely to get an answer here unless someone knew of an existing mod.