Look in the advanced setting on your wireless router.
It has the ability to have a MAC address log file.
This file is used on only allow access from devices who's MAC address is listed in the log file, and that file is protected by your control panel password on the router.
You first need to determine the MAC address of each of the devices you want to allow onto your network, and add those to the log in the router control panel.
Once your activate the MAC Address level security, on top of the WEP or WPA you should already be using, the ipod would not be able to connect, even if they new the pass key for the network.
Only devices who's MAC address apears in the log would be allowed to log onto the network.
Virtually all routers have this capability, but it is not used unless you have a situation like yours.