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first of all, check your router. some routers have a feature to throttle the bandwidth. if you can do it within your router, that'd be the easiest way.

if your router doesn't have this feature, you need some extra work. you need to build your own router/dhcpd/proxy using another pc. a x486 is good enough. install linux/bsd on this pc. use iptables/pf to set up your own firewall to throttle the incoming traffic, or use delay pools in the squid proxy to limit the download bandwidth. some linux live CDs have these kinda features built-in already, google to find more info.

maybe the first one is your way to go.