You can also disable Lightbox from being used entirely, by removing its script request from the showflat template. This will affect all of your posts, including your galleries/gallery posts.
in /forums/templates/default/showflat.tpl
FIND AND REMOVE THIS LINE:
(it is toward the end of the template)
<script src="{$config.BASE_URL}/ubb_js/assets/jquery.lightbox-plus-rotate.min.js"></script>--
CODE MODIFICATION DISCLAIMER:
source: /forums/index.php
// You may make modifications, but only for your own use
// and within the confines of the UBB.threads License
// Agreement (
https://www.ubbcentral.com/terms.php).
//
// Note: If you modify ANY code within your UBB.threads,
// we at UBB Systems cannot offer you support.
// Thus, modify at your own peril
