How to deny request from unauthorized domain pointing to my IP address?

[Cameron_D]

Last time this was brought up the easiest solution was to enable SSL, that will cause the wrong domains to either redirect to the correct one, or show an SSL error:

Set up HTTPS support with Let's Encrypt

This guide is only for existing installs where HTTPS is not enabled. Following the official setup guide automatically enables HTTPS by default. So you’d like to add https to your Discourse absolutely free, courtesy of our friends at Let’s Encrypt? Is everything else on your site ready for HTTPS? Before you start, please bear in mind that for HTTPS to work properly, every single resource on the page must be HTTPS compatible. Consider your CDN, your social logins, your…

If thats not an option you can use a pups template to add that 403 block into the Discourse nginx config.