Make proxy settings per-machine (rather than per user)
Computer Configuration\Administrative Templates\Windows Components\Internet Explorer
Applies proxy settings to all users of the same computer.
If you enable this policy, users cannot set user-specific proxy settings. They must use the zones created for all users of the computer.
If you disable this policy or do not configure it, users of the same computer can establish their own proxy settings.
This policy is intended to ensure that proxy settings apply uniformly to the same computer and do not vary from user to user.