I'm not by my console right now to tell you the exact settings, but in order to be able to FTP, etc to your PS3, while staying off the internet all you need to do is to change your default gateway address to a bad value. Your default gateway is an access point to another network - in this case the internet.
Say you are on 192.168.1.X network and your PS3 is 192.168.1.10 and your default gateway is 192.168.1.254 (your router). All your internet connections go through the default gateway, so if you change it to a bad value, say 191.168.1.254 (191 instead of 192), all your local connections will work (to your PC), but you will not be able to go out to the internet through your default gateway.