Limiting downloads from one IP address

I administer a small network with 5 PCs connected to a Belkin 4 port wired/wireless LAN router which is conencted to a cable modem.

One user uses far more Internet MBs than the others and I am looking for (a free program) or if it costs hopefully its inexpensive – to be able to limit the user’s traffic while allowing the other users free use of the net.

Can someone suggest a solution please?

Thanks
Bruce