If you have a backup or a replacement iPhone, you could go to Settings-General-Usage and then select "Manage Storage" under the iCloud heading. After that, select the iPhone backup you want to delete under the "Backups" heading, and then select "Delete Backup" at the bottom of the screen in red letters. I *think* you can also delete an iCloud backup via iTunes, but I'm not certain. As for the iCloud website, I don't recall seeing an option there, but then again, I wasn't looking for such an option...
