I don't know if you still have this problem, but I fixed mine easily. The settings you are trying to chase down aren't in Lightroom they are standard import settings in your Windows 7 control panel. Go to Start>Control Panel>Hardware and Sound>Autoplay - in there will be two major settings that are causing, or at least caused my Lightroom to open upon adding any external USB source with media content on it.
"Pictures" needs to be set to the default device you want to use, or you can do as I did and Window's standardly does and set it to ask me everytime. This enables that popup window that asks which device and action you want to use when inserting a new USB storage device.
The same setting needs to be changed for "Mixed Content" as this will try and find all different types of media file formats compatible with Lightroom.
This should fix your problem as it did mine, I have Windows 7 and they have a lot of Auto settings that need to be changed from time to time!
Hope this helped,
Sky