I don't use nginx myself, but best I can tell you would SSH to /nginx/yoursite.com and create a new file named access.conf. I suspect that you could name it anything you wanted as long as it ends with .conf, but the example I found said access.conf.

Then in that file, add:

location /login.php {
 deny 1.2.3.4;
}

location /register.php {
 deny 1.2.3.4;
}

location /admin/login.php {
 allow 4.3.2.1;
 deny all;
}

Then restart nginx.