If you simply don't want them to access the store, then go into Preferences/Parental and under Disable click iTunes Store. They will then need to have your password entered before they can use the store. They can use all the rest of iTunes without needing to sign in. Note, if they know your current account password, you will have to change it in your account details. If you want them to use the store under their own accounts, then you can set up and use multiple accounts.