On a MacBook that connects to the Internet via wi-fi, how can I prevent one user account from accessing the Internet (with no effect on other accounts)?
Note: i want to block ALL access, not just Safari.
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.
More info on the Apple web site: http://support.apple.com/kb/HT2900?viewlocale=en_US&locale=en_US
A hardware-based solution to limit all access would be to use only a USB ethernet or WiFi adapter to connect to your Internet service. Disable all built-in adapters, and only give physical access to the USB adapter to authorized users.