How can we block visitors from certain referral websites?
ANSWER: Blocking visitors (traffic) arriving to your website from certain URLs, domains, websites, or IP addresses is best done at the application level, meaning WordPress. While technically you could accomplish this at the server level (i.e. Nginx) it would be extremely difficult to manage, and in any regard, speed and performance is not really a concern for such a feature.
In summary, there are several free plugins that can accomplish this.