Traffic Management

Hi all,
we have setup a small ISP using Canopy Radios through a cisco router.
I now want to monitor a users downloads so that I can have a data cap on the amount they can download over a month and some kind of reporting to tell the user how much they have downloaded.
Can anyone suggest how to go about this,
or any software that I can use.
Any help is appreciated.

You can monitor bandwidth usage through SNMP protocol, with applications like PRTG or MRTG or whatever snmp management tool.

PRTG does this out of the box.