how can i remove my apple id from my girlfriend's iphone 4s?

there was an initial mix-up between my old iphone 4 and my girlfriend's new iphone 4s. thanks to the good folks at at&t, when she set up her 4s it somehow was got assigned my phone number. after going to the store and getting new sim cards i thought this problem was solved. now when she tries to buy something from itunes or app store on her phone it asks for my apple id. how can i fix this, please?