Keep users with no role from viewing a WordPress site.

ban_no_role_users.php

/*
  Keep users with no role from viewing the site.
  From the WP dashboard, go to Users to find the user,
  then edit the user and change role to "no role for this site".
*/
function my_no_role_lockout()
{
	if(is_user_logged_in())
	{
		global $current_user;
		if(empty($current_user->roles))
		{
			//log them out
			wp_logout();
			
			//show an error
			die("Your account has been banned from using this site.");
		}
	}
}
add_action("init", "my_no_role_lockout", 1);