Is there a way to disable the rtMedia lightbox description/meta box for non-logged in users

Hello @Simon_Matthews,

You can try adding below code to your theme's functions.php file :

add_action( 'init', 'custom_function_hide_lightbox_meta' );
function custom_function_hide_lightbox_meta() {
    if ( is_user_logged_in() ) {
        ?>
        <style>
             .rtmedia-single-container .rtm-lightbox-container .rtmedia-single-meta {
                  display:none;
             }
             #rtmedia-single-media-container {
                   width: 100%;
             }
       </style>
        <?php
    }
}

I hope this will help you.

Thank you.