bandwidth rape

So, I'm sitting on a LAN, and someone is having too much fun with it. By too much I mean 4chan moves at snail speed.
How do I nail the fucker? The LAN got over 50 computers...
Well, I tried to find him, but so far no success.

Help?

I COULD disconnect several random cables at server room and see if that helps, but that might piss off some people.

>>2
First I have to download one, with is now bloody difficult. Anything else I can do - router side? I'm using DSL-502T.