Blocking Internet access for a particular user
Have you tried using the Parental Controls preference pane? A lot of options there, none specifically to block all internet access, but you can get to the same place by only allowing access to applications without it.


From the Apple menu, choose System Preferences.
From the View menu, choose Parental Controls.


More info on the Apple web site: http://support.apple.com/kb/HT2900?viewlocale=en_US&amp;locale=en_US