Is it possible to prevent all Internet websites, but allow just a few? (Blacklist websites, whitelist some?)

Yes, you can do this with PolicyPak Browser Router.

To do this, set up a rule (route) like what is seen here. Specify that the Internet Security Zone is set to BLOCK.

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Then, make other rules which route to the websites you want. Finally, ensure your blocking policy is LAST in the list, so all whitelist items will process before the BLOCK item.

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