A website can be blocked on any windows computer by editing the hosts file.
The hosts file is a computer file used by an operating system to map hostnames to IP addresses.We can put names and addresses into the Hosts file so your computer does not have to ask a DNS server to translate the domain name into an IP number
In windows machine host file generally available in C:\WINDOWS\system32\drivers\etc\.In general hosts file is a ready only file so we cant update it directly,so copy the file outsite replace it after the change.
There will be one default entry will be there for local host ie 127.0.0.1 localhost
Now if you want to block the website www.youtube.com add the below line below local host entry
127.0.0.1 www.youtube.com