How to restrict user access to internet ?

[Mark Niven <mniven@trendline.co.il>]

Hello all!

I have a single box running sid with effectively always on broadband internet 
access through ethernet and adsl modem, using pppoe. I need a simple way (not 
for example squid which would use a lot of resources unnecessarily) to 
disable access to the internet for some users (that is smaller children) 
whilst not making it more difficult for others (my wife), such as by 
requiring manual login/logout each time. 

I'm sure there's an obvious answer to this but I have not found it by 
searching either the mailing list archives or wider google searches. Help 
would be very much appreciated.

Mark Niven