Cannot remove Scheduled Tasks created using Preferences

Greetings,

The scenario is the following:

• Domain with Windows 2003/2008-based Domnain Controllers.
• WinXP/Win7 clients
• Scheduled tasks was created using preferences (http://social.technet.microsoft.com/Forums/en-US/winserverGP/thread/2099e8da-ba89-4743-80f5-a80436d5f1ae/). The scheduled tasks run at user logon.
  

The problem is:

• When I remove the GPO from the OU where the user accounts reside (preference settings at user level) the scheduled tasks run at user logon.

I have tried the following:

• Re-apply the GPO to the OU and delete the scheduled tasks from the GPO. Result: the problem remains.
• Re-create the scheduled tasks in the GPO, and configure the action to "Delete". Result: the scheduled tasks don't run at user logon, but still exists in the client and cannot
  

The question is:

What is the process to delete completely the scheduled tasks from the clients?

Thanks in advance!