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.
This is what the Nautilus->Edit->Preferences dialog looks like. There's no Media tab: