block a website on a PC.

No is the short answer, not without 3rd party software, you're better off blocking it on your router or firewall.

You could edit the Host file with the domain name and point it to 127.0.0.0 or something.

You can block the website through your HOSTS file.

https://helpdeskgeek.com/how-to/block-websites-using-hosts-file/

Alternatively, there are software solutions available (both free and paid) that can block websites for you.

This is one of them:

http://www1.k9webprotection.com/aboutk9/protect-myself

have you done ipconfig /flushdns


Also, only put in  http://x.x.x.x

Depends on how smart your users are.

Adding the IP address of the offending site to hosts will block access to the site via URLs of the form http(s)://aaa.bbb.ccc.ddd

However, it won't block access if the site is accessed by its hostname.  For that another hosts entry is required.

Further, if the offending site is located on a server farm, or if it uses dynamic DNS, its IP address can change without notice.

But if the users are at all inclined to be uncooperative or are even a little bit clever, they'll go out and access the forbidden site through an open proxy, of which there are hundreds so they are impractical to block, or use a VPN, and any of these will bypass hosts, firewalls, routers and filters.

OK you cant block an IP address with a hosts file? (the clues in the name chaps its for resolving hostnames to IP addresses)

Are your users going to an IP address?  

if so put a static route on them to drop the traffic to that IP, e.g. if it was http://123.123.123.123

Then from an elevated command prompt

route add 123.123.123.123 mask 255.255.255.255 127.0.0.1 -P

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If that does not work, change 127.0.0.1 to be an IP address on your network that ISN'T the default gateway.
Then punch your users and put them on all a verbal warning

Since the computers are on a domain, I'll assume that you have Active Directory installed and that the DNS address that you PC's use is the address of the Domain Server as it's primary. In your DNS Manager on your Server, open the Forward Lookup Zone under your DC and create a New Zone. Name the zone the website you wish to block and don't create any records for it. Now all query to that website will be redirected by your server, to nowhere, hence blocking that website.

https://social.technet.microsoft.com/Forums/windowsserver/en-US/f53fde7c-4f48-469b-b678-92cd66737fbc/quotblockquot-a-specific-domain-in-windows-dns?forum=winserverNIS

If you are attempting to block an IP address, the Firewall is the appropriate place, since it will faithfully dump all traffic with a deny rule.

You can't block a web site through a host file using http://www.blah.com.  You CAN block an entire internet name though.  It won't JUST affect web sites.  It affects FTP, Ping, and everything else internet based from going to the target site.  In MOST cases, this is fine.

In short, DON'T make your host file entry something like these (THESE DON'T WORK!)

127.0.0.1 http://www.blah.com
127.0.0.1 http://www.blah.com:port/xxx/yyy

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DO make it something like this (THIS should work)

127.0.0.1 www.blah.com

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You can also block it for the entire domain by creating the zone for blah.com in your DNS. Then your DNS will think it should provide resolution for that domain and since nothing's defined will be unable to find blah.com

I'd still use Windows Firewall to block the IP address on the identified PC's..

https://superuser.com/questions/1159401/using-windows-firewall-to-block-a-specific-ip-on-windows-10

If you have sophisticated users, I would use a GPO to ensure no tampering could occur.