Hi Simon,
What method you can use would really depend on your account type. If you have a shared account you can use a .htaccess file to block IP addresses and ranges (you can also use this if you have a VPS account). This will still cause some entries in log files but they will get an error of access denied. A good guide to using .htaccess files to block IP address I think is:
http://www.javascriptkit.com/howto/htaccess5.shtmlIf you have a VPS account you can actually use iptables. This however is a more advanced way and if you're not familiar with it I wouldn't recomend trying it as it's possible to create a bad rule that would prevent yourself or possibly anybody from connecting to your account at all. There are many guides on this if you do a search in search engines however as stated, unless you're already familiar with it it may not be the way to go.
Hope this helps!