Google Reader is one of many applications that people use to check RSS feeds. If it’s RSS that you want to disable, just deleting wp-feed.php would make quick work of it. I suspect this is what you’re asking.
If you are really trying to ban one particular reader but still use feeds–which I don’t think is what you mean, but I’m not sure–check your server logs to see what IP Google Reader is using and ban it via htaccess. Replace the x’s with Google REader’s IP:
order allow,deny
deny from xxx.xx.x.x
allow from all
