This applies only to websites that use the default WordPress sitemaps. Replace 317 and 318 with the proper Category IDs.
/* Remove Specific Category from WordPress Sitemap */
add_filter('wp_sitemaps_taxonomies_query_args', function( $args ) {
$args['exclude'] = isset( $args['exclude'] ) ? $args['exclude'] : array();
$args['exclude'][] = 317;
$args['exclude'][] = 318;
return $args;
}
);