Look in the policy kit editor for the policy on mounting removable storage.
You could also look at your udev rules. comment out a rule used for mounting usb storage devices and replace it with one that branches to the end target.
I'm not an Ubuntu user. They use something similar to systemd. If there is a targer for automounting external disks, you may be able to disable it. You will need to use Google for this one, or wait for another response on this one. This option may not exist.
I was going to recommend blacklisting the usb-storage module, but on my system it isn't used after mounting a pen drive.
You could also blacklist the vfat and ntfs and ntfs-3g kernal modules to prevent these filesystems from being used.
Last edited by jschiwal; 06-04-2012 at 07:18 AM.