How can I remove old SMB connections?

It depends on how this was set up to launch. From boot, Go > Connect to Server > (click the dropbox) and select clear recent servers, if not using any. Then go to System Preferences > Accounts and see if there is something set up in the Login Items tab related to the server and clear. Restart and test. If it continues I'd check Macintosh HD > Library > LaunchDaemons for a file related to mounting the server.

Here's how I solved this problem - the scorched earth method - find all preference files with the server name and remove them.

Replace server_name with the name of the server giving you grief, replace file_name with the name of the preference files you find:

1. in a terminal:

   cd ~/Library/Preferences/
   sudo grep -R "server_name" .
   

2. for each file returned:

   mv file_name ~/Desktop/
   

3. restart

4. rejoice