disable admin username

[kidsguide]

By default I think the admin username should be blocked to use. It is too big of a security vulnerability if someone uses the admin username.

[jorbin]

usernames are not private information in WordPress. This is a deliberate design decision. Therefore blocking the use of any specific username doesn't provide any noticeable benefit towards security. As of WordPress 3.7, the admin username is no longer the default, new installs need to purposefully choose admin as the username and people doing so should be assumed to know what they are doing.