find:

echo '<input type="hidden" name="quickbanboard" value="' . $_GET['banboard'] . '"><input type="hidden" name="quickbanthreadid" value="' . $ban_threadid . '">';

change to:

echo '<input type="hidden" name="quickbanboard" value="' . $_GET['banboard'] . '"><input type="hidden" name="quickbanthreadid" value="' . $ban_threadid . '"><input type="hidden" name="quickbanpostid" value="' . $_GET['banpost'] . '">';