Comment # 5 on bug 949808 from Stefan Seyfried

maybe I can do something.
Can you try the tunar-volman package from obs://home:seife:xfce?

This will just prevent mounting, since the "device icon on desktop" is
apparently handled in thunar already.

If udisks (not udisks2) is installed, it will use the
UDISKS_PRESENTATION_NOPOLICY=1 udev property, if not it will just resort to
reading /sys/block/$dev/removable and not mounting if it contains "0".