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>
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