Best way to block internet on PC

If you have a edge firewall (or router) with content filtering capability then create rules or whitelist\blacklist in it to block all but the 1 site.

The following are less reliable as they could be bypassed. 
You could use things like family filters and only whitelist the one site.

Another method I seen but never tested is using a GPO to force a fake proxy then add exclusion for the site you want to allow. Unfortunately that can be bypassed by using a different browser.

I found a nice free software called K9 Protection.

just a note for you:

K9 Web Protection (From Wikipedia, the free encyclopedia)
K9 Web Protection is discontinued content-control software developed by Blue Coat Systems. In 2016, K9 Web Protection was acquired by Symantec as part of the company's purchase of Blue Coat Systems.

In April 2019, Symantec announced that K9 Web Protection would be discontinued and would no longer be made available for download or purchase. Technical support for the software ended on June 30, 2019.

K9 Web Protection Alternatives

Hi,

Before installing and trying to rely on the K9 Protection software you have found as a possible solution, you should be aware that it is no longer updated or supported. Ownership was acquired by Symantec and all technical support for it ended on June 30 of 2019.

The official “end-of-life” statement from Symantec:

“We’re immensely grateful for the loyalty of our K9 Web Protection customers. For years, many of you have been using the award-winning K9 application to protect your computers, children, and organizations from spyware, malware, adult content and other threats and risky interactions. We’ve worked together to make the Internet a much safer place for our families and communities.

On August 1, 2016, Blue Coat, Inc. (K9’s parent company) was acquired by cybersecurity leader Symantec™.  As can be imagined Blue Coat and Symantec had a handful of similar products and unfortunately, it didn’t make sense to maintain two competing products. it was decided to “end-of-life” K9 Web Protection and focus corporate and customer attention on Symantec’s line of Norton™ products.

Effective immediately, K9 Web Protection is no longer available for purchase or download.  Technical Support for K9 will end on June 30, 2019.  Until then, support for K9 can reached via email at DL-ECS-K9TechnicalSupport@symantec.com.  We’ve also created an uninstall tool should you need help removing the application from your computer.

That said, if that type of software appeals to you as a solution, then you can look at a list of alternatives (some free, some not) on the following page:

• K9 Web Protection Alternatives

I've never had a need to use this type of software solution myself so can't provide any recommendations, but the above page should give a good starting point to look at alternatives..

Hope that's helpful.

Regards, Andrew

As with many things IT, the answer is it depends on what you have, what it's worth to you, the user skills, and existing protections in place.

You COULD set a hosts file entry for the desired web site and remove DNS entries in the network configuration.  This would prevent the lookup of every other site.  Problem with this approach is that the computer CANNOT be part of a domain (domains rely heavily on DNS for simply things like logging in and mapping drives).  Additionally, if your users have admin rights to the system, they can easily add back DNS servers, "fixing" the issue.

You can use a business grade firewall that allows you to block IPs by device.

You could potentially remove the computer's gateway and just put a static route to the one network containing the website (this solution has similar issues to the first one).

You can use software that is typically used to prevent children from accessing unauthorized web sites, but you would likely have to buy that

There are likely other solutions as well.

We use a firewall or antivirus for this purpose.

OK i did the host file and it works great!!