Why do you think they would be hurting your site?
It looks like the content of the PreviewDNS site is just a verbatim mirror of your real site. Google will (even assuming that they aren't already aware of what PreviewDNS is) detect it as a mirror of your content, and will simply de-rank it appropriately.
Note that, if the existence of a mirror site like PreviewDNS could hurt your ranking, any malicious black hat SEO could achieve the same effect just by creating a similar mirror somewhere. This would be a really blatant and obvious exploit, and one Google would pretty much have to fix ASAP.
The only (more or less) real risk would be that Google might get things backwards and think that the PreviewDNS site is the "authentic" one, and that your real site is actually a mirror of it, rather than vice versa. This seems extremely unlikely to me, given the various clues about which site is the real one, not the least being the fact that the PreviewDNS copy links to your real site, while your real site, presumably, does not link to previewdns.com.
If you want to be absolutely sure that Google will correctly detect which of the sites is real, the fix is simple: just add a rel="canonical" link to the header of each page, pointing to the URL of that page on your real site. Since this link will also appear in the PreviewDNS copy, it will unambiguously indicate to Google that the copy is not the canonical location of your site. As an extra bonus, this will also transfer any PageRank accrued by the PreviewDNS mirror, should anyone actually link to it, to your actual site.
In fact, looking at the source code of your pages, it looks like you're already doing this. So you should have absolutely nothing to worry about.