Whenever you login into database server through management studio the server name and username gets saved automatically. Management studio does this for future DBA's/Developer's/Programmer's logging in reference .

The screenshots below shows how sql server management studio saves database info :-


Now you might require to delete the saved credentials.
To delete the Credentials :

• Go to the file location in the Windows 7 system's hard drive :-
  C:\Users\pablo *\AppData\Roaming\Microsoft\Microsoft SQL Server\100\Tools\Shell
  Note :- (*)  is subject to change according to the login name by which you logged into windows.
  
  
• The directory should contain a file named as SqlStudio.bin
• Delete the file and you having successfully removed the Saved Server names and usernames from your SQL Server Management Studio.