Internet Sharing - Need to restrict users once exceeded data limit

Hi,
I have a simple problem: Our ISP allows us 3Gb Downstream data per month. Once we exceed this limit, Our speed is capped to a pittiful 28.8kbps (works out around 2 to 4 Kb/S - This is a Cable connection that is capable of >300Kb/S).

One user on the network keeps pushing us over the limit. I need a way to track data usage from a certain IP address, And when a limit is exceeded, Deny Access or Cap the speed, Just the same as the ISP does.

Does anyone know of any software that will do this?

I currently use SYGATE OFFICE MANAGER to share the Internet Connection from a WIN98 PC (Old P-166, 16Mb RAM, 2Gb HDD).
Sygate keeps track of everyones data usage - If I SEE a user hogging the data allowance, I can easily blacklist them from any protocol I choose OR simply blacklist from the firewall, But at the speeds we can recieve, A user can easily push us over the limits while I'm not there - By the time I find out, It's too late

Any help greatly appreciated.