Pretty sure this isn't possible. If all you wanted to do was backup, why not just get an external hard drive? Pogoplug is meant for sharing and accessing files remotely...if you have no interest in doing either of those things, then disconnect your Pogoplug and just attach the HDD directly to your laptop.

My guess would be that you could probably do this with packet filter rules at the firewall level. Run wireshark or something similar to see what packets are being sent/received when accessing the pogoplug across the external interface of your firewall and then block that with packet filter rules. This should still allow for all normal LAN operations but block access to/from the pogoplug from the WAN.