Have you tried to disable Lock Workstation in the registry?
NOTE: *Recommend getting permission first since this can be considered a hack.*

This can be done by opening regedit.exe and go to:

HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionPoliciesSystem

If the above path does not exist you will have to create it.

You will need to create a new DWORD 32-bit value named DisableLockWorkstation and give it one of these values:

* 1 - Disable Lock Workstation
* 0 - Enable Lock Workstation

The changes should be immediate, no need to restart anything.

If the above does not work you will have to disable Fast-User-Switching feature. When the Fast-User-Switiching is enabled, other users needs to Lock this user to start a new session and login. So it is meaningless that we deny Lock permission while FUS is enabled.
The procedure to disable Fast-User-Switching is described at:

[Microsoft Disable Fast switching][1]

Procedure to disable fast switching:

1. Click the Start button Picture of the Start button, type gpedit.msc in the search box, and then press Enter. Administrator permission required If you're prompted for an administrator password or confirmation, type the password or provide confirmation.
2. Under Computer Configuration, click the arrow next to Administrative Templates, and then click the arrow next to System.
3. Click the Logon folder.
4. Double-click Hide entry points for Fast User Switching Properties, click Enabled, and then click OK.

[1]: http://windows.microsoft.com/en-US/windows7/Switch-users-without-logging-off

I just set the screensaver ot 'None' and uncheck the 'On resume, display logon screen'. That way it will never lock and the screensaver won't interfere with QTP.