Best way to prevent default server?

Define a default_server that returns an HTTP 444 code:

server {
    listen      80 default_server;
    server_name _;
    return      444;
}

(Returning a 4xx error code means requests can be interpreted by a client as an unsuccessful request, rather an HTTP 200 Blank Page But Totally Worked Trust Me.)

Just define default vhost that will point to directory with blank index.html file.

server {
    listen       80 default_server;
    server_name  _ ;
    root /var/www/placeholder ; 
    index index.html;
}

and place blank index in /var/www/placeholder

why not just deny all

server {
    listen       80 default_server;
    server_name  _;

    location / {
        deny    all;
    }
}