If you are a victim of this kind of treatment, what do you do? First, if the allegations are false, you could consider suing for defamation. It's tough to track down anonymous internet posters and tough to prove damages (how do you measure the phone calls you are not getting, for example?). There have been successful lawsuits, often at a high cost, when the poster has defamed you with knowledge that what they are saying is untrue. Opinions, on the other hand, cannot be prosecuted. It's rarely a crime in the US to have or state an opinion, no matter how abhorrent.
You could strike back, fighting fire with fire. Most professional reputation managers (formerly called "PR People") would discourage this. Demanding a retraction can often have the same effect: you only reiterate the bad news in order to retracted half-heartedly. See here.
What many companies consider as a proactive part of a healthy marketing campaign can also be a crisis-management tool: Search Engine Optimization. Firms like www.reputation.com will, for a price, manage the results of searches for and about you and your company so that the bad news appears less damning. It doesn't come cheap, and on the marketing side it may be useful to have an in-house person (a staff member, a college intern, or family member) promoting your firm with regular blog postings, news updates, and fresh content. But when something goes horribly wrong, it might be time to call a professional. They cannot remove the negative content, but they can frame the results in a way that helps the perception of your firm match the reality.