Linux – How to block flooding traffic to Nginx

I have this simple Nginx configuration:

server { 
    listen 80;
    server_name example.com;
    rewrite ^(.*) http://www.example.com$1 permanent;
}

server {
    listen   80;
    server_name www.example.com;
    access_log /var/log/nginx/www.example.com.access.log;
    error_log /var/log/nginx/www.example.com.error.log;

     root   /var/www/example.com/;

    location / {        
        index index.html index.php;
    }

    location ~ \.php$ {        
        include /etc/nginx/fastcgi_params;
        fastcgi_pass  127.0.0.1:9000;
        fastcgi_index index.php;
        fastcgi_param  SCRIPT_FILENAME  /var/www/example.com/$fastcgi_script_name;
    }
}

it is the time to prevent flooding(dos) attacks. I would like to implement those two rules:

1) I would block more then 3 requests per second for PHP requests.

example:

www.example.com/page.php
www.example.com/page.php
www.example.com/page.php
www.example.com/page.php (blocks it stop if it is within the second)

2) I would block more then 3 request per second for the same resource

www.example.com/img.jpg
www.example.com/img.jpg
www.example.com/img.jpg
www.example.com/img.jpg (blocks it stop if it is within the second)

My goal is to completly blocks the requests, I say that because i saw nodelay param returns 503 to attackers. In this case if the limit is reached I surelly know that it's an attack, so I want to block the response. With "block" I mean that I do not want to send 503 message, I want that NGINX drops the connections without sending nothing.

How can I optimize this configuration to implement these rules?

Thank you.