I had developed this some time ago when i was using a limited MB plan.
Right now it needs to be run in priviledged credentials (specially on Windows Vista, where it can not work if you are not running it as admin). When your quota is over, either daily quota or the total account quota, it pops out info boxes from tray. You can also set the warning levels in terms of MB remaining.
If you are interested, you can simply replace the message popping code with a WMI script to disable the network interface. This is not a very sophisticated code or tool, and you might want to add some more things like communication between all 3 of your machines so that a global usage counter is maintained, or else you can divide the max usage accordingly among 3 machines.
Hope it helps.
Download
A network traffic monitor. Put it in your statrup folder and give admin rights. works silently. Shows you current and average upload/download speeds, account usage display.