How can I remove a "shared" drive from my Mac?
After much investigation, ultimately this worked for me:


Go to OSX/Preferences/Sharing/
Toggle 'File Sharing' on-and-off in the sidebar
Go back to Finder and see what happens


This seems to force a refresh in Finder, removing old stuck Shared computers. How they get stuck, I don't know. In my case it happened after buying a refurbished Mac and transferring admin rights to my new local user account. The stores' old network computer got stuck in my Shared list.

Some solutions out there involving Finder/Preferences, only serve to hide the unwanted shared computer from the sidebar, not actually removing it.