You can set filters on your network interfaces to block ports, including RPC. This can cause problems of course because it blocks it for everyone, including the server itself. A better way of securing your server without adding any hardware or software is by using IPsec. You can set rules up for denying port access based on source and destination Addresses.
If nothing else, get your servers behind some kind of firewall, either software or hardware. I can't begin to tell you how full my router logs are from infected IIS servers, and netbios attacks.