At the moment we are on a Mobile Dongle so we are limited to 15Gb a month (we are changing to hardwired broadband) so data use policing is vital in our household.
We had similar issues in that both of the kids thought that ALL the 15Gb allowance was theirs exclusively so would attempt to use it all up in one go to effectively deny the other one, nice of them we thought – ALL their electronics were removed for 2 months, they were confined them to their rooms for a week and grounded for two (with only “books” for entertainment eeeeeeugh!), will admit we got sick of them moping about the house to they were encouraged out about a week and a half into the grounding.
Just as an aside and I know it’s not just ours either but why do teenagers seem to want to constantly hound adults for entertainment??? When we were teenagers the less contact and adult attention we had with adults the better…..
The simple answer this issue we found is a very simple change the password (both adults know the sequence) for the router every couple of days on a rolling basis, and monitor the connection status like hawks and lock the dongle/router down to only allow recognised devices access to the network.
We also limit the times they can use electronics this is simple enforced by the expedient of removing the dongle when we are not in the house.
It takes 2 mins as for the adults we only have our laptops (our iPhones have unlimited 3G) to connect. Everything else can plug into our lappies (kindles, iPods, etc).
The Kids seem to think that they will get unlimited when we get wired broadband but we like the peace and silence so much (they are now out a LOT) that this regime will continue.