Removing your account is not possible. From their FAQ at http://www.xfire.com/faq/#249:
‘Currently there is no User option to cancel an account. Like many other free services on the internet we do not recycle accounts.
Here’s one thing you can do to protect your info before discontinuing your use of the program. In your Xfire client go to Tools | Options | Privacy tab and uncheck “Show on my profile and miniprofile.” This will hide your extended profile information, gaming history and friends list from other users browsing the website.’
Uhuh… So that’s how they treat your personal data at Xfire, they just tell you to hide it!