Give user option to select NA/ Leave the rating for a particular category.

function.php

/**
 * Allow to remove method for an hook when, it's a class method used and class don't have variable, but you know the class name :)
 */
function remove_filters_for_anonymous_class( $hook_name = '', $class_name ='', $method_name = '', $priority = 0 ) {
    global $wp_filter;
    // Take only filters on right hook name and priority
    if ( !isset($wp_filter[$hook_name][$priority]) || !is_array($wp_filter[$hook_name][$priority]) )
        return false;
    // Loop on filters registered
    foreach( (array) $wp_filter[$hook_name][$priority] as $unique_id => $filter_array ) {
        // Test if filter is an array ! (always for class/method)
        if ( isset($filter_array['function']) && is_array($filter_array['function']) ) {
            // Test if object is a class, class and method is equal to param !
            if ( is_object($filter_array['function'][0]) && get_class($filter_array['function'][0]) && get_class($filter_array['function'][0]) == $class_name && $filter_array['function'][1] == $method_name ) {
                // Test for WordPress >= 4.7 WP_Hook class (https://make.wordpress.org/core/2016/09/08/wp_hook-next-generation-actions-and-filters/)
                if( is_a( $wp_filter[$hook_name], 'WP_Hook' ) ) {
                    unset( $wp_filter[$hook_name]->callbacks[$priority][$unique_id] );
                }
                else {
                    unset($wp_filter[$hook_name][$priority][$unique_id]);
                }
            }
        }
    }
    return false;
}

function remove_actions_for_anonymous_class( $tag, $class_name = '', $method_name = '', $priority = 10 ) {
    remove_filters_for_anonymous_class( $tag, $class_name, $method_name, $priority );
}

if (class_exists('WPJMR_Edit_Review')) {
    remove_actions_for_anonymous_class('add_meta_boxes','WPJMR_Edit_Review','add_meta_box', 10);
    class WPJMR_Edit_Review_Ext extends WPJMR_Edit_Review {
        public function __construct() {
            parent::__construct();
        }

        public function output( $comment ) {
            wp_nonce_field( 'wpjmr_save_data', 'wpjmr_meta_nonce' );

            $review_categories = get_comment_meta( $comment->comment_ID, 'review_categories', true );

            foreach ( $review_categories as $category_slug => $category ) {
                $current = get_comment_meta( $comment->comment_ID, 'star-rating-' . $category_slug, true );
        ?>
        <p><label for="star-rating-<?php echo esc_attr( $category_slug ); ?>">
            <?php echo esc_attr( $category ); ?>:

            <select name="star-rating-<?php echo esc_attr( $category_slug ); ?>">
            <?php for ( $i = 1; $i <= WPJMR()->wpjmr_get_count_stars(); $i++ ) : ?>
                <option value="<?php echo $i; ?>" <?php selected( $current, $i ); ?>><?php echo $i; ?></option>
            <?php endfor; ?>
            <option value="NA" <?php selected( $current, 'NA' ); ?>>NA</option>
            </select>

        </label></p>
        <?php
                }
        }
    }
    new WPJMR_Edit_Review_Ext();
}

if (class_exists('WPJMR_Form')) {
    remove_filters_for_anonymous_class( 'comment_form_top', 'WPJMR_Form', 'can_submit_review', 10 );
    class WPJMR_Form_Ext extends WPJMR_Form {
        public function __construct() {
            parent::__construct();
        }

        /**
         * Comment form stars.
         *
         * Add stars to the comment form based on review categories. Done via action hook.
         *
         * @since 1.0.0
         */
        public function comment_form_stars() {

            ?><div id='wpjmr-submit-ratings' class='review-form-stars'>

                <div class='star-ratings ratings'>

                    <?php foreach ( WPJMR()->wpjmr_get_review_categories() as $category_slug => $category ) : ?>

                        <div class='rating-row'>

                            <label for='<?php echo $category_slug; ?>'><?php echo function_exists( 'pll__' ) ? pll__( $category ) : $category; ?></label>

                            <div class='stars choose-rating' data-rating-category='<?php echo $category_slug; ?>'>
                                <?php for ( $i = WPJMR()->wpjmr_get_count_stars(); $i > 0 ; $i-- ) : ?>
                                        <span data-star-rating='<?php echo $i; ?>' class="star dashicons dashicons-star-empty"></span>
                                <?php endfor; ?>
                                <input type='hidden' class='required' name='star-rating-<?php echo $category_slug; ?>' value='NA'>

                            </div>
                            <?php if ('food' == strtolower($category) ){
                                ?>
                                <span>&nbsp;&nbsp;<?php _e( 'Leave the ratings no food is there.', 'wp-job-manager-reviews' ); ?></span>
                                <?php
                            }
                            ?>
                        </div>

                    <?php endforeach; ?>

                </div>

            </div><?php
        }
    }

    new WPJMR_Form_Ext();

}


add_action( 'wpjmr_after_save_comment_review', 'wpjmr_after_save_comment_review', 10, 2 );

function wpjmr_after_save_comment_review($comment_id, $comment) {
    if ( ! $review_categories ) {
        $review_categories = WPJMR()->wpjmr_get_review_categories();
    }

    $review_average    = 0;
    $$counter = 0;

    foreach ( $review_categories as $category_slug => $review_category ) {
        if ( isset ( $_POST['star-rating-' . $category_slug ] ) ) {
            $value = $_POST['star-rating-' . $category_slug ];
            $review_average += $value;

            update_comment_meta( $comment_id, 'star-rating-' . $category_slug, $value );
            $review_categories_value = get_comment_meta( $comment_id, 'star-rating-' . $category_slug, true );
            if ($review_categories_value == 'NA')
                $counter++;

        }
    }

    if ( $review_average > 0 ) {
        $review_average = $review_average / (count( $review_categories ) - $counter);
        $review_average = round( $review_average * 2 ) / 2;

        // Save the average
        update_comment_meta( $comment_id, 'review_average', $review_average );

        // Save review categories in database for this review.
        update_comment_meta( $comment_id, 'review_categories', $review_categories );
    }
}