Read the readme section on the --usb command. You can specify how many devices to mine with and which specific device types and usb slots if you like.
Eg:
--usb :1
will limit it to mining only on the very first usb device it finds that it supports.
If you're talking GPU mining then you're in the wrong place.
Nah, not gpu. Just got a bunch of ASICMINER block erupter usb's and a few of the antminer usbs. I don't want cgminer touching the antminers because it has no real support for them. I looked through docs, i'm just tired of reading them. Didn't see that in there, for some reason. thanks