How to Enable, Disable or Rename a Login in SQL Server

To disable a login in SQL Server use the following command:

ALTER LOGIN [login_name] DISABLE;
GO

disable_login_sql_server

…to enable it:

ALTER LOGIN [login_name] ENABLE;
GO

enable_login_sql_server

To disable a login:

ALTER LOGIN [login_name] WITH NAME = [new_name];
GO

rename_login_sql_server

If you need to Enable, Disable or Rename the 'sa' login read the related blog post:
Enable, Disable or Rename login SA in SQL Server