Adam,
There is NO permanent way that you can make a Windows machine unable to see a particular web site AND be unable to restore visibility as long as the person in question has admin rights and even limited internet search abilities. Even if you deleted some things, there would be the opportunity to do a "Windows Repair" that would restore the integrity of the system. At BEST, you can make this blocking condition occur in a very obscure way that would make the person have to work very hard to remove the blocks. This is the old security principle of the "low hanging fruit." You are in essence making the fruit hang higher. But if the person wants it badly enough, they WILL get to it.
As others have advised you, this is the sort of thing best managed by an external network layer physically and logically outside of the machine in question. Admin rights can override anything you can do. You can only try to revoke the person's admin rights. THEN you might have a shot. But if you did that to me, for example, I would haul your ... anatomy out on a platter and roast you. That kind of invasion is GUARANTEED to tick someone off major big-time, and perhaps even invite retaliation.
Post #3: "one machine only. windows 10. no corporate environment. a kid's machine."
Post #9: "you're missing the point. I'm already smarter than you are by you thinking this has anything to do with a kid."
Adam, if you can't give us accurate descriptions of the REAL problem, why should we even talk to you" You have in the past claimed to be Catholic, which means the "big 10" are part of your to-do list. So if you really ARE Catholic, STOP with the "bearing false witness" stuff. Though to be honest I wouldn't be surprised to see you now deny that religion.