Prevent default server or wildcard in nginx

I have two domains: staging.abc.com and www.example.com pointing to the same server IP in my DNS. I don't have a site enabled/available for staging.abc.com, but I have one for www.example.com. However, whenever I access staging.abc.com, it points to www.example.com.

Here are my confs. My /etc/nginx/nginx.conf is default. I didn't change anything.

I don't have a sites-available/default. It's been removed.

# sites-available/example
server {
  listen 80;
  server_name example.com;
  return 301 http://www.example.com$request_uri;
}

server {
  listen 80;
  listen [::]:80;

  root /home/deployer/example;
  index index.php index.html index.htm;

  server_name www.example.com;

  location ~* .(jpg|jpeg|png|gif|ico|css|js|woff)$ {
  expires 30d;
  }

  location / {
    try_files $uri $uri/ /index.php?q=$uri&$args;
  }

  error_page 404 /404.html;

  error_page 500 502 503 504 /50x.html;
  location = /50x.html {
    root /usr/share/nginx/html;
  }

  location ~ \.php$ {
    try_files $uri =404;
    fastcgi_split_path_info ^(.+\.php)(/.+)$;
    fastcgi_pass unix:/var/run/php5-fpm.sock;
    fastcgi_index index.php;
    include fastcgi_params;
    fastcgi_read_timeout 300;
  }

  include /home/deployer/example/nginx.conf;
}


# nginx.conf
user www-data;
worker_processes 4;
pid /run/nginx.pid;

events {
        worker_connections 768;
        # multi_accept on;
}

http {

        ##
        # Basic Settings
        ##
        charset utf-8;
        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 65;
        types_hash_max_size 2048;
        # server_tokens off;
        proxy_read_timeout 300;

        # server_names_hash_bucket_size 64;
        # server_name_in_redirect off;

        include /etc/nginx/mime.types;
        default_type application/octet-stream;
...

I tried inserting this to the http block but to no avail:

server {
    listen      80;
    server_name "";
    return      444;
}

Here's what I want to achieve:

1. Changes are preferably done on sites-available.
2. The server should not resolve any connection (returns 444) for all domain names that have not been assigned to any app.

Unless you explicitly define a default server, nginx will use the first server block with a matching port for any request where there is no server_name match. See this document for details.

You should create a catch all server block, for example:

server {
    listen 80 default_server;
    return 444;
}