Does anyone know how to limit bandwidth usage (For example, 5GB/Monthly) to a user account on the system with IPFW? The opreating system I am running is FreeBSD. I read on some tutorials that you can do it with pipeline, but I don't understand it exactly.

I appreciate your help.

Does anyone know?

give more info.
The customer have dedicated IP?
pipe can work only if the user have dedicated ip.
Also you can limit the transfer rate. So you can config the server so this domain canot use only 1mbit transfer.

IPFW with dummynet will allow you to limit the bandwidth rate of the connection. For example you could limit the bandwidth to 256Kbit/s on a particular IP.

Given that your user is on a dedicated IP:
You will have to play around with the bandwidth rate to account for ups and downs in your users traffic patterns. If you are allowing 5 GB/month of total transfer, you have to give your user enough bandwidth to transfer that amount of data.