One thing I did in he past when I was managing internet for an office is install a caching proxy server.
So what this does is it keeps local copies of many things so that they don't need to be downloaded again.
How much it helps varies.
Realistically, though, this is not a paid job for which you have all day. So I would say start by talking to them.
You need to know exactly what the problem is, where the bandwidth is going. I don't know if you do. It could even be you neighbors stealing from your connection.
Then if that does not work you can try to install an appliance between your modem and network, that can do quota control, or upgrade your router to one that has such features. I am guessing there must be routers/ access points where you can do things such as set quotas and control access.
aliessayli2 wroteno country has the quota problem on dsl as we have in lebanon
That is true, but there are solutions. In most routers, you can go in the administration interface and see a listing of bandwidth consumed by each IP.