Also, dependent on your environment you can just turn off SQL Server Authentication whatsoever.
There are two types of authentication supported by SQL Server - Windows Authentication and SQL Authentication - you can have both or one of them active. If you switch off SQL Authentication then only valid Windows (as defined in the system) users will be able to use the server (normal permissions still apply, so each account has to be added to SQL Server as well, the fact that somebody has an account in the system doesn't mean they can access the SQL Server instance).
You can configure this in the server settings from the Management Studio.