There might be a 3rd party application you can install on the computers directly to limit their throughput, however this may also limit their throughput on the local network for file sharing/print sharing which may really suck.
Beyond that the way its done is usually through QoS as the poster above said, though I do not believe the airport extreme has any advanced abilities for QoS. You might considering looking into getting a router that you can flash to dd-wrt which is a open source router software that gives many advanced features that most routers software do not support from the manufacture. Or you if your into learning a bit you could get one of the business grade routers though that can be quite a bit to dive into if you do not have any prior experience.