Google is starting to support this natively in the search results via the "Block results from $foo" link, but it seems they're rolling it out slowly so you may not have access to it yet. See [this article](http://googleblog.blogspot.com/2011/03/hide-sites-to-find-more-of-what-you.html) for more information.
In the meantime you can use a browser plugin to accomplish this.
<https://chrome.google.com/webstore/detail/nolijncfnkgaikbjbdaogikpmpbdcdef>
<http://www.optimizegoogle.com/about/>
[Chrome](https://chrome.google.com/webstore/detail/nolijncfnkgaikbjbdaogikpmpbdcdef) <br />
[Firefox](http://www.optimizegoogle.com/about/)
--jeremy