Add those sites to the hosts file. Presuming the OS is an NT derivative, it’ll be in c:\%windir%\system32\drivers\etc. If it’s Win95/98/ME, it’ll be in C:\windows. For each site you want to block, add a line like this:
Use the real URL, of course, not the ‘pornsite.com’ in the example. Save and exit. Make sure whatever text editor you’re using doesn’t add an extension to the file name. It has to be just ‘hosts’, with no .doc, .txt, or .rtf. Also make sure the user doesn’t have permission to un-edit the file.
This causes that user’s PC to resolve the naughty URL to its own IP address (localhost). If the user is knowledgeable enough, however, they can ‘ping’ that URL from another (unrestricted) PC, note the site’s IP address in the response and use that IP address in the (restricted) PC’s browser rather than the URL. Not perfect, but at least it’s free.
is a freeware (that I can’t vouch for) that promises to do what you’re asking.