As amazing as Nginx is, they unfortunately do not have an easy method like Apache where you can just create a dynamic file in your site's directory (.htaccess) that nginx will parse in real time. Their flawed reasoning for that is here.

To block an IP in nginx, you have to be able to edit your nginx server configuration file. If you're capable of doing that yourself, you just need to create a simple "location" rule. You can see the structure of that in the nginx docs.