If you don't want to keep a login name any more, you should delete it by using the "DROP LOGIN" statement as shown in this tutorial example:

-- Login with "sa"

DROP LOGIN Dba_Login;

GO

Command(s) completed successfully.

-- View login names

SELECT name, sid, type, type_desc FROM sys.server_principals

WHERE type = 'S';

GO

name sid      type type_desc

-------- ----------------------------------- ---- ---------

sa  0x01      S SQL_LOGIN

ggl_DBA 0x348AF32B3B58CB40B67A5F5B7086B96E S SQL_LOGIN