IE Trusted sites - how to clean with group policy?

[Rytis]

Hello,

Does anybody knows, how to clean trusted sites before adding new ones?
We need to have only approved sites in the "Trusted sites" zone, but there
can be other sites (in the end-users PC`s) which was added by users before
new GPO take effect.
Another question is how to manage these sites? Because if we decide to
remove some sites from the list in GPO, they still remain in users PC, in
browser settings.

Thanks.

Rytis