document library permission

hi friends

i have a issue regarding permissions in document libraries. i have created a new document library and gave unique permission to certain users. i have also placed that doc lib as a web part and used target audience to that unique group. now only those people are able to see that web part. now my problem is if some other user gets that document link he is able to open that document. how to prevent that user from accessing that document? please help

M.C.Jayanarayan

Your Web site administrator may need to create a custom HTTP header that forces Microsoft Internet Information Services (IIS) to inform the users' client software that it is communicating with a server running SharePoint Team Services. To create a custom HTTP header, the Web site administrator needs to follow these steps:

1. On the Windows taskbar, click Start, point to Programs, point to Administrative Tools, and then click Internet Service Manager.

2. In the right pane, right-click the computer name, and then click Properties.

3. In the Master Properties section, click WWW Service and click Edit.

4. Click the HTTP Headers tab.

5. Click Add.

6. In the Custom Header Name box, type:

   MS-Author-Via

7. In the Custom Header Value box, type:

   MS-FP/4.0,DAV

8. Click OK.

9. Click Add.

10. In the Custom Header Name box, type:

    MicrosoftOfficeWebServer

11. In the Custom Header Value box, type:

    5.0_Collab

12. Click OK.

13. Click Apply. If the Inheritance Overrides dialog box appears, choose one of the following options:

    • Note which servers have unique settings applied to them and manually add the "MS-Author-Via" and "MicrosoftOfficeWebServer" headers to each virtual server later.

      or

    • Click Select All to apply the settings you just made to all your virtual servers. If you choose this option, you will lose any custom headers that you added to your virtual servers.

14. On the Windows Taskbar, click Start, point to Programs, point to Accessories, and then click Command Prompt.

15. In the Command Prompt window, type:

    IISRESET

    and then press ENTER.

Reference :

http://www.sharepointusecases.com/index.php/2010/03/configure-item-level-permissions-for-document-libraries-part-2/

http://technet.microsoft.com/en-us/library/cc722851.aspx