The WordPress Content Management System supports multiple users with different levels of authentication and capability. For many websites the user interface for Administration is not so important as the look and feel offered to end users.
But if you are the website owner or administrator then you will need to be able to login to the site in order to make changes.
In the absence of any user interface solution simply use the website URL with /wp-admin e.g.
In any content management system, if you offer the opportunity for visitors to register then you will we soon get annoyed by the amount of spam you get. This site is no exception.