This gave me quite a hard time, so I share the solution here :

By the way, to "su" easily without giving up sudo, you can create an alias in your normal user's .bashrc

alias su='sudo su'

With this alias you can now su or su - just like in a normal Unix, and you keep the advantages of sudo without the pain in the ...

And now for something completly different.

First click on the partitions shown in gnome so that udev mounts them, and have a look at how they are seen by the kernel (what you see with sfdisk is different so it is important to look in /etc/mtab), then right click unmount the partitions and create a new rule 95-hide-ntfs.rules in /etc/udev/rules.d containing this (replace sdc1 and sdc2 according to what you found out in mtab) :

ACTION!="add|change", GOTO="hide_partitions_end"
SUBSYSTEM!="block", GOTO="hide_partitions_end"
KERNEL=="loop*|ram*", GOTO="hide_partitions_end"

################################################## ############################

# Partition sdc1 that we want to hide from gnome
KERNEL=="sdc1", ENV{UDISKS_PRESENTATION_HIDE}="1"

# Partition sdc2 that we want to hide from gnome
KERNEL=="sdc2", ENV{UDISKS_PRESENTATION_HIDE}="1"

################################################## ############################

LABEL="hide_partitions_end"

Once this is done and your are sure that the partitions are unmounted (no longer present in /etc/mtab) issue :

udevadm trigger

And your ntfs partitions have disappeared from Gnome