Thanks for your reply.
There is a one word answer to your question: money.
If you sue the complainant to remove the negative comments, then you will be shelling out a lot of money on legal expenses. You may ultimately win, but regrettably, even if you win, your adversary may not be able to pay the judgment amount. Many defendants simply file bankruptcy and avoid payment (though proof of defamation may satisfy the "willful and malicious injury" requirement necessary to avoid a discharge of a defamation judgment in bankruptcy).
Alternatively, you could simply offer the complainant money to take down the complaint and sign a settlement promising not to make disparaging comments about you or your business activities in the future. Of course, this is a very unsatisfying option, but as it is an option, I can't avoid mentioning it to you. It all boils down to the value of your good name. Sometimes the value is great enough to justify paying off your opponent.
Those are really the only options to completely remove the negative comments. You can also try to flood the website with positive reviews, if they accept that type of commentary. That's how businesses organizations that sell rehabilitation against negative review services handle the problem.
Note: federal law (47 U.S.C. 230(c)) provides absolute immunity to websites that post negative user reviews. This law was originated to protect the internet during its infancy. In the traditional print publishing world, any republisher can be sued for defamation, in the same manner as the original commentator. Until Congress changes the law to reflect the reality of the now matured internet, the problem that you are suffering from will continue to persist -- because lawyers, such as myself, are effectively prohibited from suing large websites for defamation, so as to stop them from this sort of unrestricted public negative commentary.
I hope I've answered your question. Please let me know if you require further clarification. And, please provide a positive feedback rating for my answer (click 3, 4 or 5 stars) -- otherwise, I receive nothing for my efforts in your behalf.
Thanks again for using Justanswer!