ISP-like bandwidth/download limiting

Hi,

I manage a network with several users. Each user has his/her own pc, and
uses that for internet access. Each PC has a fixed IP.

Is it possible to impose a download limit based on computer (IP address),
just like most ISPs do? For instance, if the user on PC 1 has used up his
download limit (e.g. 10GB/month), bandwidth for that user (PC) should be
limited to smallband (e.g. 64k). What (GNU) tools can I use to achieve this?

Tnx!
E.T.