Catch-all for Port 80 (http://)

server {
   listen 80 default_server; 
   server_name _;
   return 404;
}

Catch-all for Port 443 (https://)

First generate a self signed cert:

openssl req -nodes -x509 -newkey rsa:4096 -keyout self_key.pem -out self_cert.pem -days 3650

server {
    listen 443 default_server ssl;
    ssl_certificate /path-to/self_cert.pem;
    ssl_certificate_key /path-/self_key.pem;
    return 404;
}