Internet Data Restriction By User Account?

While I was at school there was a system in place where by you had to enter a user name and password to access the internet. Every student had a data limit like 3GB per month for example. I remember it had something to do with a proxy. I would like to recreate this system on my office LAN as some staff members have been downloading a lot slowing down the (very expensive) Internet connection. Limiting each users data will discourage large downloads.