to block a user:
right click on my computer --> manage --> users and groups --> local users
just right click the user and pick "disable"
to block a site:2 options
1.
start --> run --> notepad %windowsdir%\system32\drivers\etc\hosts
in the file write the folowing text at the bottom:
www.walla.co.il 127.0.0.1
that was an example incase you want to block "www.walla.co.il" site
2.
open internet explorer
tools --> internet options --> security --> untrusted sites
click on "sites" button and just add the sites you don't want to be shown on that computer.
I think it works only of internet explorer... so don't catch my word on it.
and I hop you don't mind the english, didn't quite figured on how to install hebrew fonts on this redhat...