Disable a specific wireless interface in Ubuntu?

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http://serverfault.com – I have a Ubuntu 11.04 running with two wireless interfaces -- one onboard (seen as wlan0) and one external card via USB, seen as wlan1. I would like to only use the USB card, completely ignoring the onboard one. Currently when I enable wireless both interfaces come up. How do I make Ubuntu ignore wlan0? (HowTos)