How to make a website inaccessible without showing any message

Not sure what you're after here or why but there are a number of tricks you can pull depending on a number of factors.

Is this a 3rd party website over which you have no control? If so then you'll need to rely on routing. I might consider intercepting the URL and redirecting it quietly to Google.

Thanks Russ. Can you direct me to instructions on how to re-route? Basically the owner of the company is having his kid intern in the company and he wants to block the gaming etc. websites the kid normally uses, so he is not distracted - but not so that the kid knows he is being blocked. Thanks.

OK, that simplifies things a bit since it sounds as though your boss is willing to block the websites company-wide rather than just for a single user. How you do it depends on what equipment you're using. The first question is, how is traffic routed to the Internet? What are the devices between your workstations and the Internet that you might be able to use to intercept requests?

If you're using Active Directory DNS, you can probably add some CNAME entries there. Your router hardware might have something that performs URL redirects. The quick & dirty approach is to just modify the hosts file on each local machine. This is by far the easiest to do on a single workstation but it would take some time and it's difficult to manage long-term.

It's a very small office and he has his own connection in his office - directly from cable company - separate from other workers. No firewall device or other equipment except Verizon router and apple airport device . The son will be using a Win10 machine in that office.

OK - got it. Would you me able to tell me specifically what I would need to put in the hosts file to block a particular site?
Thanks for your help!

Thanks Russ!