how to disable to automatic wireless interface up in ubuntu? [closed]

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http://serverfault.com – On my laptop I have a built in wireless card and I got one external connected via usb. By ifconfig command I saw both the cards connects to internet and gets IP. eth1 - the interface my laptop has in-built wlan1 - I have connected externally via usb. Now I applied "sudo ifconfig eth1 down" so that i can use only wlan1. But this eth1 goes down for sometime and then come up again automatically. So I am not able to test my externally usb connected wifi adapter. Can anyone suggest me way to disable eth1 interface? (HowTos)