A WebmasterWorld thread named I've banned Googlebot discusses the best possible way to remove a site (your site, not a competitor, hopefully) from the index. Many of the members in the thread simply recommend disallowing Googlebot from indexing your site with the robots.txt file. Others also recommend using the remove URL request form.
But many warn that if you do this, your site will be gone for at least 6 months from the index. So if your rankings drop and you think its due to a ban, do not jump to disallow Googlebot right away.