Help disabling Windows Updates on an existing fielded asset

Hello,

We are trying to block our users who are currnently in the field from downloading Windows updates from Microsoft.  We can block these through the image using a group policy but need a way to block them on existing users PCs without requiring us to reimage them or manually touch them.  Has anyone written a script or program of some kind which will prevent the user from downloading Windows updates from Microsoft?  Is there a policy or something i can enable to do this for us?  

We are running Altiris CMS 7.5