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This a tutorial for basic host booting (DoS):http://www.modernmoding.com/index.php?topic=183.0The only problem with that is it will only boot them from the game, if you're wanting to keep their internet down (DDoS) you will need to spread a virus onto other peoples computers that will all DoS the IP simultaneously.
You can grab thier ip using cain "ARP POISONING", once thier IP has been located you can do a simple mass UDP flood on that ip on port 3074 which is a xbox live port, after about 60 UDP pings the host xbox 360 will freeze kicking them off live till they restart...... Or you can get thier IP and if its DSL you can tell them what thier house looks like using GxxGLE EARTH "input latitude and longitude" and it will show you a arieal of thier house..... NOTE: that only works for DSL users since its a phone line...... so scare them and then they will leave lol