We provide the ability to block requests on a per-site per-IP basis at the edge of our network. Using this feature is much faster than blocking IP addresses in your .htaccess file.
By default, all accesses are allowed. We maintain two lists for each site, an allow list and a deny list, which are processed in the order: allow, deny. Thus, an IP address that appears on both lists will be allowed. Entries to either list can contain either an IP address or a netblock specified in CIDR format (10.20.30.40/24).
You can see and manipulate your site's IP access controls (if any) in the "Access Controls" box on the Site Information page.
To see exactly what blocked visitors see, visit blocked.nfshost.com, which blocks all visitors.
See also What do I do if someone is trying to waste my site's bandwidth?