I found this posted in the macrumors.com forums as a possible solution
"Since the Shared column uses a sort of metadata to find the name of the connection, the name is stored onto the list. But it is supposed to delete it once the connection is terminated. As it was said in an earlier post, you have to go to the finder prefs. Before you do this, you need to terminate your internet connection, and turn off airport. Then you go to Finder>Preferences>Sidebar and uncheck Connected Servers, Back to Mac, and Bonjour Computers. This should reset the Shared list and the Computers should no longer be there."
My interpretation.
1. Turn off wi-fi or unplug ethernet cable to terminate network connection
2. Go to Finder Preference and uncheck (disable), Connected Servers, Back to Mac, and Bonjour Computers
This should reset the Shared list in Finder Sidebar
3. Go to Finder Preference and check (enable), Connected Servers, Back to Mac, and Bonjour Computers
4. Turn on Wi-Fi or plug in ethernet to re-connect to network