WiFi connectivity on iPod touch
"Forget" WiFi networks you don't want to automatically join.

Your iPod touch will automatically join any network that it has previously joined.  In order to prevent it from jumping on any of these network automatically, you must "forget" them.  Go to the wifi settings, tap the little blue arrow to the right of the network you want to prevent your device from joining, and then at the top, tap, "forget network". (exact wording?)

Hope this helps.