I am not 100% familiar with ripoff report, but I read that any company can post a rebuttal to the supposed claim right underneath the ripoff claim. The only way that I know to get the page de-indexed would be to set up a 404 not found page which obviously you can't do. So why not take the option of filing a rebuttal that soothes the situation? Don't attack the person that filed the claim, but instead demonstrate that your client's business is capable of reconciling these types of claims. If you did that, anyone that visits the ripoff report would see that your client's company had taken the time to remedy the issue. That just seems like the more logical approach.

RipOffReport is a PR6 site, so trying to spam it to deindexation would be nearly impossible.

Much better off posting a rebuttal as noted above.

You could also build some web properties on different domains(web 2.0 posts, blog posts etc) about your client, and then SEO those to rank above RipOffReport. That's how the reputation management firms do it.

Incorrect. There have been cases where blackhatters have targetted sites with a TON of porn links and links from known "bad neighborhoods" to sites they wanted to bring down. Now, there's a bunch of factors that go into it, but it has been possible.

Ohh my, what a scam! Get a negative report about some major company ranked highly enough and extort them for thousands to get it taken down!

It is easy to deindex a YOUNG site. If you tried to spam an established site like that you would probably just end up pushing it higher in the SERPS.

You could create a review site of your own and attempt to rank that site above the rip-off-report.