If using Gnome, uncheck the following checkbox in gconf.
Or try running this as root (also if using Gnome):
gconftool-2 --type bool --set /apps/nautilus/preferences/media_automount false
If you're not using Gnome, you could try editing /etc/fstab if the device is listed there, not an expert on messing with that file though.
Here's the Google results: