What I would do is first contacting the webmasters of sites where negative comments are posted and complain about them.
If you can't (or they don't help you), try making a WHOIS lookup to get the details of the buyer(s) of the Internet domain name where the negative reviews are. To do that, you can for instance visit http://www.register.com/whois.rcmx, then write the domain name(s), click "go" and follow the instructions. You will have the postal addresss, phone number, etc. of the people who bougth the name.
If you still have problems, try contacting the companies that host those sites. They have to help you!.
As a final recommendation, as You proceed with these, try cleaning your reputation, making others to review your business. If it is good, in the end it will standout by itself.

Regards,

David
Web developer at BuzzyVenture.com