Block Inheritance

HI All,

 I have following OU structure and want to stop applying one GPO.

    -Computers

                 -SiteA ( all Win7 PC's)

                     -Windows XP

 I add one GPO to SiteA  This GPO is installing application to these computers but not compatible for windows XP.

So how do i block or stop applying that One GPO.

Can WMI to allow that?

AS