How do you change the password to prevent purchases in iTunes without changing your iTunes password
How do you change the password to prevent purchases in iTunes but not change your Apple ID password
Your Apple ID (assuming you only have one Apple ID) is used for all Apple services and only has one password. See Change your Apple ID password - Apple Support
including the following:
Your Apple ID is the account you use to access Apple services like the App Store, Apple Music, iCloud, iMessage, FaceTime and more.