Assuming you're wanting to block an IP address, then there's a couple of ways of doing it:
1) Generate the RSS feed dynamically - e.g. using PHP or Perl - and check the "REMOTE_ADDR" (IP address of remote user) environment variable.
2) If you have admin control of your web server, then use the built in functionality of the web server software to block the IP address (e.g. use the deny directive in Apache).