Thank you for your question.
You can remove it using Yoast SEO plugin https://wordpress.org/plugins/wordpress-seo/ as described in the following page.
You need to add following code in the robots.txt file.
The Disallow directive in robots.txt is about crawling, not indexing.
You probably want to implement a meta robots noindex element or the X-Robots-Tag; noindex HTTP Response header.