This is a common problem without a simple solution, assuming you're trying to monitor every device from a central location. You can't monitor these kinds of stats from a single PC, you need to get in the middle of everything, your router.
I've been in similar situations with bandwidth-hogging roommates, I was able to flash my router with the Toastman Tomato Firmware which will allow you to monitor and restrict bandwidth usage on a per-device basis (note: the standard tomato firmware won't break down bandwidth usage by individual device.)
If you want to go this direction, the link provided explains the process from flashing the firmware to setting up monitoring.
It is a rather involved process, and you need to make sure your router is compatible.
Assuming your using wifi what is the make/model of the router? – cybernard – 2014-07-11T02:04:12.957
Cellphones usually have built-in record keeping. – cybernard – 2014-07-11T02:07:25.517
4If you could load an open firmware such as ww-rt or tomato on your wifi router, it would be relatively easy. – cybernard – 2014-07-11T02:10:46.220
The easiest solution would be to Just upgrade your broadband package :-) – rfsk2010 – 2014-07-11T13:12:59.357
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This is closely related to http://superuser.com/questions/533361/office-internet-monitoring-usage
– Brian – 2014-07-11T15:37:11.823@cybernard: Is ww-rt a thing? Googling it turns up a lot of dd-wrt hits and nothing with the text "ww-rt" on the page. – user2357112 supports Monica – 2014-07-12T08:36:53.523
@user2357112 Yes, I meant. ddww-rt and tomato are alternative firmware that allow your router to run linux. Then you have most of the linux tools for monitoring,firewalling, and etc instead of the few options provided by the manufacturer. – cybernard – 2014-07-12T15:43:11.230