how can I prevent nautilus from running when I plug in a USB stick?

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http://askubuntu.com – I run XFCE4 on Ubuntu 11.10. When I plug in a USB memory stick, I get two new windows popping up, both showing the contents of the memory stick. One is Thunar (which I want) and one is nautilus (which I don't). Before I plug in the stick, nautilus isn't running at all. Something is spawning it, but I don't know what, or how to stop it. (HowTos)