Carlos

In many WordPress installations is possible enumerate usernames through the author archives, using urls like this:
http://wpsite/?author=1
http://wpsite/?author=1/
http://wpsite/?bypass=1&author%00=1
http://wpsite/?author%00=%001
http://wpsite/?%61uthor=1
And recently wordpress since 4.7 comes with a rest api integrated that allow list users:
curl -s http://wpsite/wp-json/wp/v2/users/ 
curl -s http://wpsite/?rest_route=/wp/v2/users 
curl http://wpsite/?_method=GET -d rest_route=/wp/v2/users
Know the username of a administrator is the half battle, now an attacker only need guest the password. 
This plugin stop it.
Also, is possible get usernames from the post entries. 
This plugin, hide the name of the author in a post entry if he is not using a nickname. 
Also, hide the url page link of an administrator author.
The main goal is hide the administrators usernames. 
Obviously, is better not choose “admin” as the username because is easiliy guessable.

No User Enumeration

mattturow

This plugin checks for common user names that don’t exist in the users table. If the submitted user name meets settings criteria the IP will be blocked from the site.
The plugin also has the ability to stop author enumeration by bots.
<h4>Recommended Settings</h4>
There are two different ways to protect your site from user name brute force attacks. The more strict way is to immediately block an IP that attempts to login with a non-existent user name. This can possibly cause issues with websites that have many users or users that may not login very often and forget their user name. It’s most useful for one-person websites.
The less strict way is to have a black list of user names that only a bot would try like “admin” and user nicenames that aren’t logins.
The “Stop Enumeration” setting redirects all attempts to ?author={ID} to the home page of your site. This will stop bots from incrementing through your users and discovering user nicenames.

Double Knot

Stop brute force login attempts by user name.

No User Enumeration vs Double Knot