So use a poste restante billing address, then, and never change it. Last I checked, picking up one's mail on a regular basis is not required by iTunes Ts and Cs, nor must a credit card remain on file to keep an iTunes account active.
There is, although via iTunes and it is a little subtle. It may also depend on your country (but Australia and Peru work).
1. In iTunes, go to your account (iTune Stores, then click on top right login name assuming you're logged in).
2. Edit your Payment information
3. Choose CC type of "None"
4. Save
It is a desirable thing to do if you expect children will be using your iDevices -- the password doesn't stop them buying more $$ stuff after you've just authorised them buying something for free. And I'm assuming you didn't type your password in front of them...
I guess didn't think of going through iTunes to remove the credit card I had to add to get the free XCode app via the Mac App Store.... yes, the whole setup is completely insane. ;)
