WiFi Management and security ??

During the summer months, I live in an RV park where we have Wifi.

we are having issues where we are almost exceeding our monthly bandwidth [17gb per month] so the ISP has been shutting us down.

I was talking to the park Mgr today, seeing that we are at 85% of our usage.... and he is starting to think that someone is tapping into our signal.

I'm not really that knowledgeable with network stuff.... but the gist of our situation is ... we have 435 owners in the park.... weekends do tend to get busy to where there maybe 15 laptops logged on [some days more some days less] ... but week days maybe 3. I do think we have a problem with kids gaming on weekends nights... but i can't prove it.. .

anyhooo.... we need some solution to 'lock' out unwanteds.....

either password protected, but i'd like some sort of networking software who we can see who is accessing the signal and even if we could track the bandwidth they are using.

can anyone shed some light on this and possibly recommend a software?

Thanks Bunches....

Terese