Delete offline files with group policy

DCs: Windows 2008 R2
Clients: Windows XP with SP3

I want to delete all the offline files cached locally for all the clients (Delete both the temporary offline versions and the versions that are always available offline). There are too many clients to delete manually one by one. I want to use the group policy to implement. The following procedures are what I did.

1. In the Group Policy Management Editor, Computer Configuration -Policy - Administrative Templates -Network -Offline Files, Enable At logoff, delete local copy of user's offline files.
2. On a client, gpupdate /force
3. Reboot the client computer and then log on to the domain
4. Log off
5. Log on to the domain again

However, the offline files still exist.