How to block a specific user from logging in?

To do this, you can use WPJobster User Ban plugin.
A workaround solution, without using the plugin:
  1. Go to your WP Admin Dashboard.
  2. Click on “Users”.
  3. Select the specific user and press the “Edit” button.
  4. Change user's email.
  5. Click on “Generate Password”.
  6. Click on “Update User” button (bottom of the page).