I think NETLIMITER is an answer here. You can install it on the other machines and see what they are doing and the rest is their use.

But GAMING alone can push you over the cap. Or using youtube, hulu and more. And we didn't start to include downloading!

I too was approaching the 5GB wireless internet cap and had to forbid all video sites. When you hit the cap, just turn it off or agree to pay for it.
Bob