How do I disassociate my iTunes account with an iPhone?

Don't let the other person sync their iPhone with your iTunes library on your computer.

The iTunes account that is available on an iPhone is transferred to the iPhone via the iTunes sync process - with the iTunes account logged in to with iTunes when syncing.

This person needs to log in to their iTunes account with iTunes on their computer and sync the iPhone with his/her iTunes library. This will erase all existing iTunes content on the iPhone along with your iTunes account.

Click on the iTunes store where the little shopping bag is on the left. Then you will see your username on the upper far right side above the music and other contents ads and all that other stuff. Click to sign your account out. Now their should be a popup window so that the other user can sign in with their own username and password, and create an iTunes account. They can plug in the new iPhone and activate it or update it or add music to it or whatever you want to do with it.

Be sure to sign them out and sign back in with your own name and password before syncing your own iPhone and iPod so that your phone number and backup information gets put on your phone. Be careful about keeping things straight about which phone is signed in when you hook them up.