How do I disable the user login block
I had a similar problem.

As described in the issue, we had the same problem in Drupal 7 with the "No front page content has been created yet" message. In Drupal 8, if you have no content on the page the default is to display the user page. If the user is not logged in, it defaults to the login page.

The best solution to this is to have content on the front page. However, assuming you want a blank front page except for some custom blocks, you can remove the Login Form by either creating or editing the existing 
page--front.html.twig file in your theme. 

Since you don't have any content on the front page, you can remove the "Content" region and then remove the "Header" region to remove the tabs. 

EDIT: I just found that if you do remove the "Content" region from the page--front.html.twig file, this will also remove it from the login page, which is a problem. Not sure why this is. Still working on a solution.

Finally, make sure that you leave the "Front Page" setting blank = 'Manage > Configuration > System > Front Page'

These are the two blocks I removed from my theme

   {# Content #}
    {% block content %}
      &lt;a id="main-content"&gt;&lt;/a&gt;
      {{ page.content }}
    {% endblock %}

  {# Header #}
  {% if page.header %}
    {% block header %}
      &lt;div class="col-sm-12" role="heading"&gt;
        {{ page.header }}
      &lt;/div&gt;
    {% endblock %}
  {% endif %}