I intend to write something to do this soon, at the request of my wife. Basically, you need a cron job or something similar that edits /etc/hosts - shouldn't be too difficult.
But it would work better for people who don't know enough to manually edit /etc/hosts.
EDIT: the /etc/hosts thing would be to block specific sites, but it's just as easy to take the internet connection down completely with ifconfig or whatever your system uses.
