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Log into the user account you want to restrict.
Go to "Internet Options" and click on the "Connections" tab. Click on the "LAN Settings" button and put a tick in the box next to "use a proxy server for your LAN". Type into the "Address" box:

127.0.0.1

Click "OK" then "OK" again.

This will make the users account you need to restrict unable to access the internet.

You can use a Group Policy setting to remove the "connections" tab in "internet options" to prevent the settings being changed back.

Click "Start" > "Run" then type:

gpedit.msc

Click Computer Configuration > Administrative Templates > Windows Components > Internet Explorer > Internet Control Panel > Disable the Connections page, and click Enabled, click Apply then OK.

This is easily reversable.