Clear Drop Down List of Recent Connection From SQL Server Management Studio

SQL Server Management Studio 2017 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\14.0\SqlStudio.bin

SQL Server Management Studio 2016 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\13.0\SqlStudio.bin

SQL Server Management Studio 2014 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\12.0\SqlStudio.bin

SQL Server Management Studio 2012 delete the file C:\Users\%username%\AppData\Roaming\Microsoft\SQL Server Management Studio\11.0\SqlStudio.bin

SQL Server Management Studio 2008 delete the fileC:\Users\%username%\AppData\Roaming\Microsoft\Microsoft SQL Server\100\Tools\Shell\SqlStudio.bin

SQL Server Management Studio 2005 delete the file – same as above answer but the Vista path. C:\Users\%username%\AppData\Roaming\Microsoft\Microsoft SQL Server\90\Tools\Shell\mru.dat

In fact there's a way to delete all the history info in the dialog fields.
For SQL Server 2005:
C:\Documents and Settings\<user>\Application Data\Microsoft\Microsoft SQL Server\90\Tools\Shell\mru.dat
If you can not find mru.dat at above location look for mru.dat in following folder.
C:\Documents and Settings\[user]\Application Data\Microsoft\Microsoft SQL Server\90\Tools\ShellSEM\mru.dat
For SQL Server 2008:
C:\Documents and Settings\<user>\Application Data\Microsoft\Microsoft SQL Server\100\Tools\Shell\SqlStudio.bin
If you can not find mru.dat at above location look for mru.dat in following folder.
C:\Documents and Settings\[user]\Application Data\Microsoft\Microsoft SQL Server\100\Tools\ShellSEM\mru.dat
If you are using VISTA OS instead of XP OS. Replace C:\Documents and Settings\[user]\Application Data\Microsoft\ with
C:\Users\\AppData\Roaming\Microsoft\ and it should work.
Make sure to take note down any IP address of SQL Server you may need in future.
Reference : Pinal Dave (http://www.SQLAuthority.com)