SQL Server Tip: Remove Windows Authentication on SQL Server

You cannot disable Windows Authentication on SQL Server. In fact, SQL Server recognizes two modes of security:

1. Windows Authentication
2. SQL Server AND Windows Authentication

There's a way out, however. Users of the Administrators group of Windows are able to connect to the SQL Server because there's a login account in SQL Server for it and is assigned the System Administrators role. You'll need to delete that login. But before you do so, please be sure that

1. The security mode is set to SQL Server and Windows, and 
2. You know the password of SQL Server's sa login 

Otherwise, you'll render your SQL Server inaccessible To do so, follow these steps:

1. Open SQL Server Enterprise Manager 
2. Expand the nodes to reveal your SQL Server instance 
3. Expand your SQL Server instance node to reveal the various nodes 
4. Expand the Security node and click Logins 
5. Right-click the BUILTIN\Administrators login and click Delete, click Yes when prompted for confirmation 

No one from the Administrators group will be able to access your SQL Server now.

Referred:
http://social.msdn.microsoft.com/Forums/en/sqlsecurity/thread/cc3339b8-24f4-4e5a-ae2f-fd491e93864d