You can't.
Change your iTunes (Apple ID) password along with any other password that was stored in the iPod. If any passwords are associated with credit cards, contact the CC company and get your card replaced (with a new number). If any passwords are associated with a bank or any other savings institution, contact them also and discuss approprate action with them.
The only real protection you have is with the personal information on the device rather than the physical device itself. This requires action before the device is lost/stolen. If the device has significant personal information, it should have a strong 8-digit (or longer) password AND be configured for automatic wipe in the event of ten consecutive incorrect password entries.