To block bots by User-Agent in Nginx, add the following to the server entry of the website.
if ($http_user_agent ~ (semrush|opensiteexplorer|ahrefs|seekport|mj12bot)) {
return 403;
}
Restart Nginx
systemctl restart nginx
To block acces to a specific file or folder, use
location /file-to-block.php {
return 408;
}
If you are using Apache web server, see How to block Bad Bots (User Agents) using .htaccess or Block User-Agent using Cloudflare