Part of the SEO process is taking into consideration the pages you want people to see, If you have a large site or multiple web sites you can influence which pages on those sites show up in rankings by placing links to the pages you wish to rank higher. If you wish to completely suppress a page you can make sure all links going to that page contain a "no follow" tag.
There has been some recent confusion regarding the "no follow" tag because there was an announcement by Matt Cutts of Google that these tags are being ignored when used within sites for the purposes of page sculpting. Some have taken that to mean that the "no fallow" tag is being ignored completely. I am of the opinion that these tags are still in effect when the link is pointing to another website.