Before you do resinstall, what makes you think pieces are left behind?
Sometimes an uninstall will leave a module and you'll see, for example, the icon even after the application delete. If something like that is the case, simply go to options > advanced options > applications > modules and delete the yahoo go module.
Otherwise, see blackberryfaq.com for instruction on reinstalling the OS.
Here is link: Main Page - BlackBerryFAQ.com