Forget login from connection drop down menu
In SSMS 2008, I believe you need to delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\%Version%\SqlStudio.bin

This will, however, clear ALL of the logins and saved servers.  You may be able to edit the file using an editor, but I believe it would just be easier to start from scratch.

If you are using SSMS 2012 or greater, you can simply delete the server from the dropdown and that will allow you to start fresh with just that server by highlighting it in the dropdown and hitting Delete

See below link for reference to Aaron Bertrand's answer:

https://stackoverflow.com/questions/10993861/how-to-remove-cached-server-names-from-the-connect-to-server-dialog