Eugen Bobrowski

Plugin provide to help search engines to indexing site correctly.
A simple plugin to manage your robots.txt. Plugin donn’t create the file or edit it. This plugin edit WordPress output of robots.txt content. And get you a easy and usable interface to manage it.
Features
<ul>
<li>Drag-n-drop robots.txt paths</li>
<li>Changing <code>blog_public</code> option form plugin settings page</li>
<li>Site map field for robots.txt</li>
<li>Robots.txt physical file checking.</li>
</ul>

Robots.txt rewrite

first review

Provide the easy managment of your robots.txt from admin side. It propose you the advanced then standard robots.txt content too.

Chris Klosowski

Pretty simple. Just blocks semalt.semalt.com with a 403.
If your web host uses a web server that doesn’t support .htaccess, then this plugin is for you.
If your host supports .htaccess, you should try https://wordpress.org/plugins/block-semalt, as this is the best method.

WP Block Semalt

Simply uses the HTTP_REFERER to determine if it's from 'Semalt', and blocks it with a nice 403 "Unauthorized" response.

Robots.txt rewrite vs WP Block Semalt