Recently, a group of online reputation management experts were comparing solutions for moving negative blog posts off the first page of search results.
Asking bloggers to remove the post is one option that has worked in some cases. (The requests were courteous and/or referenced factual errors in the posts.) In other situations a cease and desist letter from an attorney worked. In those cases the posts were not consumer complaints or negative reviews, but posts considered libel or slanderous.
Cease and desist letters are not an approach for everyone, but they are appropriate for some situations. An attorney should be consulted if it is an option you are considering.
Keep in mind that increasing numbers of sites are also posting more stringent user rules. In cases where commenters are breaking them, they can be barred from using the site and their content is removed by site administrators. Resolving some issues may be as simple as an email to the site administrator.