How to Allow or Prevent New User Registration
As admin of your own site, you can decide whether to allow or disallow new user registration. If you’re the only author and user, disabling the new user registration feature is the best option. However, for a membership site, for example, you need to enable new user registration so anyone can register a new account on the site.
For any site that allows new user registration, in the login form, you will see a Register link at the bottom of the form.
In this quick tutorial, I’ll show you how allow or disallow new user registration. To do so, go to Settings > General. You will see a checkbox to allow or disallow new users registration. Untick that. That’s it.
I recommend leaving the new user default role as subscriber or contributor. I believe granting author, editor or administrator role to a user should be done manually since those roles have greater access to the site. For security reason, those three roles should only be given to trusted person. Please read about users roles and capabilities in WordPress Codex for more information about this.
Although this prevent someone from registering an account in your site, if you wish to create an account for someone else (such as guest writers), you can still do so by manually creating an account for them from Users > Add New.