You first need to install dconf editor
in Software Center. In my case it shows remove because I have already installed it

Then you need to open dconf editor
via Dash

Then goto org.gnome.desktop.media-handling
You will find automount
checkbox on right hand side. Uncheck it

If you prefer not to install anything extra in your system. You can use gsettings
to change dconf keys.
To disable Nautilus automount use this command in a terminal
gsettings set org.gnome.desktop.media-handling automount false
To enable it again open a terminal and use this command
gsettings set org.gnome.desktop.media-handling automount true