Don't allow them access to the other computers on the network, and if you actually find a virus on them consider shutting off their Internet access for some pre-defined period, say a week.
Also, I would alter the hosts file on those machines to prevent access to "bad" places. To do that you use the notepad in administrator mode on that machine (right click notepad, run as admin) and then go to the hosts file windows/system32/drivers/etc then select all file types and open the file named hosts. Paste the huge list of bad places into the file from here: http://winhelp2002.mvps.org/hosts.txt
Just copy the entire text file and copy it into your hosts file. Then save and all those sites will be blocked.
The main site gives you a bit more detail on what hosts file blocking does: http://winhelp2002.mvps.org/hosts.htm