How do I stop people connected to a wireless network from downloading large files?

I have set up a wireless network at my gliding club. The network is pretty simple - a Dell desktop computer using XP, and a D-Link DI-824VUP+ wireless router. Our internet connection is via a wireless broadband modem. We also have a VoIP phone connected.

We provide onsite internet access to club members as well as to visiting pilots, so we have not secured the network with WEP/WPA at this stage. The site we occupy is very remote, so we aren't concerned about "unauthorised" users - the nearest public road is a mile away. Nevertheless, we are having problems with increased usage - we are now regularly exceeding our monthly plan allowance. In rural Australia, with our appalling "broadband" services and high costs, that's breathtakingly expensive (try 15c per MB).

Is it possible to continue to provide our members and visitors with unsecured access to web weather and flight briefing services, webmail and similar resources, but to stop them from downloading large files (audio and video, for example)? I can't see how we could use the router's URL blocking/domain filtering settings - it just doesn't seem practical to try and identify and block sites, there are just too many of them. Restricting access using WPA is possible, but won't guarantee against misuse. Logging usage is also possible but difficult - as with most volunteer-run organisations, it's hard to get people to spend time analysing router logs when they could be out flying!