gistfile1.php

<?php
add_filter( 'optin_monster_api_output', 'tgm_om_filter_lightbox_by_tag', 10, 4 );
function tgm_om_filter_lightbox_by_tag( $optins, $optin, $fields, $post_id ) {
	
	// If in a particular tag, remove the lightbox.
	$cat_exc    = 362; // Replace with the tag ID to target.
	$slug 		= 'sjdfhsjakdkg'; // Replace with the unique slug for the lightbox you want to remove.
	$tags = wp_get_object_terms( $post_id, 'post_tag', array( 'fields' => 'ids' ) );
    foreach ( (array) $tags as $tag_id ) {
        if ( $tag_id && $tag_id == $cat_exc && in_array( $slug, $optins ) ) {
            unset( $optins[ $slug ] );
            break;
        }
    }
    
    // Return the modified output.
    return $optins;
	
}